Well Mundus is sort of like Satan, right? Just under another name, so as not to seem like a videogame was really trivializing a religion or whatever. All the hallmarks of stuff being lifted from Christianity are in there anyways. But I can make a pretty big excuse for Satan: he was probably an idiot for challenging God in the first place, so maybe he wasn't all that bright. As one of the higher angels, I sort of figure he ought to have known that God really was going to kick his ass if he tried anything, because God made everything, even him, and he can therefore more than likely unmake everything as well, just as fast. But Satan did cause a war in Heaven, so he got that far... and maybe he naively believed he could actually defeat God. Until God sent his burning ass tumbling into Hell of course.
But then, what God does, rather than just disintegrating this guy for trying to take over Heaven is he allows him to continue existing in Hell. In fact, he even allows Satan to rule over hell, or something similar, and he allows Satan to have influence over human beings - for test purposes, I guess. Satan has this "job" to do, which is tempting human beings to do bad things, given to him by God. Or at least allowed to by God. He's still God's errand boy of sorts... so he's doing the will of God too, in a roundabout way. I can't blame him for that, any more than I could blame him for being created imperfect by God himself. I could blame him for being kind of stupid in attacking God... but I could also blame human beings for being stupid eating that apple and getting us all kicked out of Eden. Angels and humans are God's creations, so they're apparently sort of like children. And children do stupid things. But we don't generally blame them for doing dumb stuff because they haven't become smart yet. Which is why I have a problem with God punishing people and all for eating one apple, when they obviously didn't know anything anyway because they were 'completely innocent' (i.e. kind of stupid or with no sense of responsibility) at that time. But that's another debate.
So, if we can make an excuse for Satan, in that he's kind of dumb (because he thinks he can defeat God... or maybe he thinks he can take over the Earth even when there's already a prophecy that he can only take over for a short time because eventually God WILL come down and throw him into the Lake of Fire). That IS kind of stupid, but if he's fated to lose and burn horribly for eternity, I guess you can't blame the guy for wanting to wreak a little havoc first. And apparently Satan is all angsty about his old defeat because he really wants to ruin God's pet project - humans. So the guy's a bit of a hothead. I can understand that too. Or if he is really being employed by God, for now, then it's not his fault if his job happens to be tricking people into doing evil stuff. It's what he's supposed to do.
As a character, Satan just seems like kind of an idiot to me in some ways. In other ways he's like this tragic favorite son figure who tries to get power and gets put in his place and eats sour grapes ever after, and sucks be to him because ultimately he burns forever for making a stupid mistake. And in other ways, maybe he's like... actually just enjoying his time because he just loves doing evil stuff? I dunno. In any case, does Satan really do evil stuff himself? Like does he personally torture souls in hell for kicks? Or does he have like flunkies to do it for him while he sits back with a beer? I get the impression Satan is just a tempter. HE doesn't do evil stuff, he just gets people to do it so he can take their soul later. If he does personally enjoy torturing people for nothing I can't much sympathise with him, but I think that's the point of the character. He's just written in the Bible as an adversary, and we're not meant to understand him. He's just there, doing evil stuff, because that's apparently his thing, that God lets him do.
Now I don't think Mundus himself operates exactly like Satan, but he's got some qualities of Satan and he was clearly ripped off of Satan and modelled on a Greek God and also an angel, assumedly so that we get the impression he's like this powerful, proud, ancient angelic-like being. And I think he has more in common with an angel than a demon when we first see him. The popular picture of a demon people have with the pointy tail and the wings and all that is probably not how Satan would look at all, since he was an angel once, and since he has the power to trick people, he probably doesn't look like a terrifying, disgusting monster. He'd probably choose to look handsome, impressive, or even endearing (like that little kid in those Omen movies), because that stuff wins people over more easily than terrifying monsters do. Mundus is doing this by appearing as a statue. He looks like a big, strong, wise god or something in two of his three incarnations in the game (excluding DmC which I will not mention because I don't like that interpretation), physically intimidating because of his size, but otherwise the sort of thing humans would drop to their knees and worship. Makes all that business of making humans serve him much easier I guess. Okay, so later he reveals he's actually a gross thing made of hands and eyeballs, but I think he could probably look like anything he wants. When his "look at me, I'm so impressive and god-like" act fails with the statue, he probably tries appearing like a giant orange booger to make Dante crap his pants.
But you can establish from all this that (classic) Mundus is proud, got kinda jilted by Sparda, and really really really wants to rule over humans, and he hates Sparda and all Sparda's kids because Sparda inconvenienced him for quite a while and then buggered off to rule the human world himself. So is this guy unapologetically evil? Well no, because I can actually understand some of these motivations. Some humans have similar motivations. It's a very human thing to desire power, you don't have to be some kind of genocidal demi-god to do it. People like exerting power over each other all the time, even if it's really subtle, like knowing more facts than your other nerdy friend does, and telling him so via the internet to "put him in his place". It's also not unusual for humans to dislike people who screw them over - betray them - as Sparda did to Mundus. I can totally understand Mundus wanting revenge on Sparda, and I can understand him wanting revenge on Dante if Dante thinks he's gonna do exactly the same thing as his father did and slaughter all my minions and creepy wooden puppets that I spent ages making.
He's proud too. But most of us are proud of something. Our views, our kids, our heritage... whatever it might be. So it's not crazy for him to be proud of somehow getting to be the Emperor of the whole entire Underworld. I'd be proud of that achievement, and getting that far... especially if I were a giant orange booger-thing. And I wouldn't want some upstart like Dante just taking it all away.
Now yeah, he did play a pretty lousy trick on Dante with Trish and with turning his brother into a zombie-like slave, and most importantly apparently kidnapping or killing Dante's real mother as some kind of revenge for what Sparda did, and later using her soul to create demon clones. But this guy is clearly furious. He's not thinking straight. He's so wrapped up in revenge and getting into the human world he does something incredibly DUMB - he INVITES the one guy who could kick his ass to the island he's about to enter the human world from. I mean that's his biggest sin of all really - extreme freaking stupidity. At least enter the human world and build up your forces first, you idiot. But like most bad guys, he suffers from pathological overconfidence and thinks he can just enter the human world AND kill Dante in one go.
I dunno. Putting aside Mundus and his revenge/ruling thing, The demons of (classic) DMC don't strike me as inherently mindless evil beings who are just running around doing evil for the sheer hell of it. If you put aside the small fry who are only there for us to trash with our swords as Dante, the demon bosses - who have some kind of character development or lines to say - well they're often just kind of cold, indifferent to human suffering, desire power a lot... but more importantly they all seem pathologically proud (and/or deluded). Sanctus thinks he can somehow take Sparda's place, even if it means trashing his town and killing a bunch of innocent people. He's a megalomaniac, not a basement butcher. Probably a bit senile, too. Vergil is kind of a megalomaniac, as is Credo, and Agnus... Arkham's got a lot of that in him, as has Arius - all the 'big bads' of the series, essentially. They're all nuts for power and glory and ruling people, which is not evil in itself. What they do to get it, though, is where they could be classed as 'evil'. Other lesser demons exhibit more admirable qualities actually. Like Beowulf, for example. He just seems really ****ed that someone betrayed him. I mean, oh the humanity! Thought must have tormented the poor guy for eons. And Griffon is a noble servant, far as I'm concerned. He was just doing what his boss told him to do, to the best of his not-so-great ability, but he came off as a guy I actually felt sympathy for because he wasn't just attacking you for fun OR power. Nothing evil about him, really. Cerberus is a proud guard-dog, doing his duty. Nevan wants to have a bit of fun with Dante (by eating his life/soul) but when she gets her ass kicked she actually decides to help him ('cause she fancied Sparda I think. Oh boy, the Freudian implications of Dante plucking away and making that guitar really sing... oh, dear me). Point is, I think the games kind of go out of their way - unlike say, Dante's Inferno (the game) - to show that demons in general have their own free will and they're not all just murderers that have numb brains. Many have personalities, desires, self-esteem, and certain social expectations. Just like people do, except demons really love and respect the whole power thing, apparently. In fact, these demons might as well be aliens, or some other life form, like Romulans, who just enjoy a good fight and letting those emotions flow, baby.
So Mundus, though he was ripped off of Satan apparently, is just like a big ol' Emperor Palpatine or something. Except better, because I could never figure out what Palpatine did any of his evil crap for, other than kicks and because Sith wanna rule the galaxy, but Mundus at least did some of it because he was trying to get back to the human world which he had been booted out of, and to get revenge on the child of the one who gave him the boot. There was a personal element to what he was doing, because of Sparda. I don't read Star Wars books so I don't know about whatever reasons the Sith need to rule the galaxy other than for pure lust of power, but if the DMC manga is canon, then the reasons for Mundus' conquest of the human world are stated there. DMCs demons don't seem 'just evil' at all to me, and the series itself takes pains - even in the title - to let you know it. Dante is part-demon, so he's like, part violent, and that violent part kinda gets off on killing his own kind for revenge... but he does it for the weak, so that's okay by us. The idea that devils don't cry is really saying that "hey, some DO cry, so don't tar em' all with the same brush, 'kay?" Sparda was a good demon, Arkham was a bad human, Dante is a good hybrid, and so on. It's basically saying all the way through that even though this whole series was conceived from pre-existing ideas about a black-and-white sort of good vs. evil thing, that demons ain't all bad. A lot of 'em have actual feelings and motivations that was can understand. And yes, we do have to understand bad guys to some degree to accept them at all. You don't have to like them or what they do... but don't forget there'd be no actual story without them, or without someone willing to screw things up to create a story. ^___________________^