Yeah, but he didn't actually kill his mom. He just left her there, and not necessarily to be taunted by demons, seeing as how he killed almost all of them.
Besides, she's a warrior, she was able was able to handle herself until the monsters overwhelmed her in the first place.
In the DMC3 manual, it mentions how Vergil always believed that Eva loved Dante more. This is just a reference to the older version of his character.
Old Vergil probably would have ended up killing (she can't be killed, so incapacitating her with his o-katana) Eva, had he met her in the demonic realm after DMC3.
These are two similar, yet very distinct levels of sickness. Old Vergil doesn't care, as long as his "image" is kept intact, and New Vergil is the victim of his own insanity. But he's choosing what he believes is the right choice, according to his own twisted logic.
Old Vergil simply made up that "power is protection" excuse as a way to keep his "honorable face" in check in front of his brother.
I don't think this comparison is valid.
I don't remember DMC3 manual mentioning this, the manga writters described Vergil this way (and the manga is not canon, and it has much inconsistences compared to what was presented in the game) but absolutely nothing in the game points this out. Even the protection thing hints that Vergil loved Eva and wants power in the first place to protect something or someone that was never mentioned or that even he doesn't know for sure (my bet is that he feels guilty for not being able to protect her back in that time). My source:
DMC Lair.
Vergil would never kill his mother or point his sword directly to one being incapable of reacting properly to his attacks, unless this being tries to kill him (while posing a real threat) or be in his way, he even spared Lady's life twice (on the ritual place and in the library) who posed one annoyance to his plans. Just looking at his lines and his way of acting you can assume this. He most ignores Lady, even knowing that she could pose some annoyance to his plans, he doesn't tried to kill her while fighting Dante (which would be an easy task for him, just seeing how easily he slayed Beowulf, the fight with Lady shows that Dante beated her with ease).
Vergil clearly takes this honor thing
as a warrior seriously. He nearly killed Arkham because he disobeys his orders to dispatch of the girl, and he knows that Arkham was not human anymore and were able to fight him. Obviously he looks much more powerful than Arkham, but Arkham was able to "defeat" he and Dante when they were exhauted from their very own battle.
But i agree that Vergil's mind is somewhat twisted in both cases. He cannot realize that even his urge for power is a "scream" of his human feelings and ego.
DmC tried to approach things much more to our contemporary reality, were peoples lacks on morals and are dirty as the characters in the game are (and i'm not saying that it's bad, but i personaly don't like it). They don't used the common archtype of a hero or even an anti-hero, they tried to bring us more contemporary humans to their story and that's why new Vergil is very different from old Vergil. Sure that they tried to make some references for the old fans to be pleased, but the new Vergil is setted in another world and is a completely different character, he has his own personality, morals, objectives and story. DMC is much more "puritan" the way they put their characters, Sparda's efforts were to be praised, equal to Dante's and the game gives you no room to question their acts as immoral or such things, Dante was killing demons that were trying to destroy the human world, he was fighting to protect the weak just like his father. While Vergil's motivations remains a mystery, but he had a lot of honor (that even Dante don't had, as a warrior).