1. There is no proof that Vergil ages like normal humans.
DMC 1, Trish says: You're the man who lost a mother and a brother to evil twenty years ago, the son of the Legendary Dark Knight Sparda, Mr. Dante.
It's been 20 years since the attack that made Dante think Vergil was lost as a child, and Dante has aged, same as Vergil. Dante has not stayed looking 8 years old for 20 years, so he must age at a relative rate to humans. Same goes for Vergil. Maybe they reach adulthood and then age slowly after that? Who knows.
2. Vergil wanted power to defeat Mundus, and we saw he warned Lady to get out of his way, instead or remorsly killing her.
Vergil wanted power to gain the legacy of Sparda, prove himself over his human side inferiority, and prove he was superior over what stemmed from a traumatic childhood over not being able to save his mother from demons.
As for Lady, he wanted Arkham to kill her.
Arkahm says :I'm afraid I should ask the uninvited one to leave. That is what you want.
Vergil says: Why didn't you kill her? Perhaps, because she is your daughter? Did some pesky fatherly love get in your way?
He wanted Lady dead and thinks Arkham is weak for letting emotions get in the way. He didn't want to spare her. At that point in the game, Vergil was weak from bloodloss and already had a fight with Dante and Jester going on at the same time. Lady wasn't exactly a priority.
3. There is no implication Vergil hated his human half
Arkham insults Vergil about it.
Arkahm says:What about you? You're an incomplete being as well. Both demon and human blood mingle in your veins.
Then Vergil tells him to 'shut up'. He doesn't like being told that he's only half a demon, half human. He sees humans as weak, and his attitude in DMC3 proves it. He wants full power, demon power without the human side getting in the way. To him, humans are weak and full of emotion that gets in the way of gaining power.
Vergil says here: Why didn't you kill her? Perhaps, because she is your daughter? Did some pesky fatherly love get in your way?
To further your study of the black arts, you sacrificed your loving wife, to become a devil as well. Knowing this I thought you'd be more useful to me, but I was wrong. No wonder your attainment of power is incomplete.
There, he's saying that Arkham's humanity got in the way of him wanting to be a demon. Therefore, to Vergil, humanity and emotions are weakness to him. He doesn't like that.
There's also this that Arkham says to Vergil : You have lost... Because you've underestimated humans.
Even Arkham knows that Vergil doesn't regard humans with much respect and thinks they are weak.
Right now, Nero's origins along with the timeline, whatever DMC2 was about, what actually happened in Dante's childhood, and whats actually canon etc is in the writer that comes next's hands.
DMC4 isnt exactly forthcoming with details and the only details we know are from sources easily discarded, so they would be free to resolve them however they want, so they could use one of theories we stated or something entirely new.
Pretty much. They didn't go into this with a solid plan, so plot points are tacked on without much explaining, leading to messes like where Nero comes from. It wouldn've helped if they'd thought it through beforehand.
I wouldn't have minded if there was concrete evidence in the game about Nero; instead they release this important information in a guidebook that is in Japanese and hard to find outside of Japan. But do we take this guidebook as canon or one of many possible ideas?