Please riddle me this, why does Vergil always get the Draco in Leather Pants treatment
a) An "explanation" is already on that very Trope's page. You've mentioned it enough times I'd have thought you'd have read it.
b) If you are wondering why people are talking about his possible goodness, or why he has popularity at all, it's partly because he isn't a "bad guy, full stop". He is a morally ambiguous character, a Chaotic Neutral. That doesn't make him a cardboard cut out in the sense Mundus is, evil for the sake of it, evil
because Satan. He is a foil of Dante, capable of both good and evil acts, or motivated by understandable reasons - at least, he was in DMC3.
It amuses me that just because he is the chief antagonist to Dante in that game he is automatically considered by some players as "evil". And Dante isn't just as vicious in his own way? Dante clearly likes killing demons, and guess what? Not all demons are bad. Where does that leave Dante, eh?
Vergil is a 'selfish b*stard' character, but not an inherently evil one. He raises a tower and wrecks a town, sure. Collateral damage to him. But the only person he actually 'murders' in that game is Arkham, because he was savvy to Arkham not being in it just to help him, and knew he was up to something and keeping things from him. Did he kill Lady when she was trying to kill him? Did he kill Dante when he almost certainly knew being impaled by a Devil Arm would awaken him? Actions speak louder than words. Of all the series' villains, Vergil is the least villainous and the least obsessed with being evil
just for the hell of it. If people died because of him, it was because he considered them expendable/inferior in the long run, and that is Vergil's sin or 'evil'. He's not a psychotic maniac, or a literally evil-obsessed individual like Arkham, nor an oppressive tyrant like Mundus. He is Machiavellian, and surely no hero, but at the same time, he's not to be lumped in with the Evil simply because he opposes Dante.
Some people find that interesting, cool, attractive, worth talking about, or whatever.