And then Vergil finds out and pretends to be Dante just like how Noah died in Final Fantasy XII and Basch taking Noah's place.
Why this? I don't really know. Maybe it's because of the thing Nero said when he activated his Devil Trigger. To me it seemed like it wasn't Nero talking alone but with Vergil. Quote goes as:
"From that day forth . . . my arm changed . . . and a voiced echoed, 'Power! Give me more power!' and if I become a demon, so be it . . . I'll endure the exile . . . anything . . . to protect her!"
This is going to be a weird interpretation, maybe's it because I'm weird. The first part of "From that day forth" and "my arm changed" is obviously Nero talking, but who knows, maybe it's Vergil as well when he activated his Devil Trigger for the first time; DT Vergil's arm-sheath. Then the voice echoing "Power! Give me more power!" could be both Vergil and a voice in Vergil's head. Now, many people acknowledge Vergil's reason for trying to become more powerful. He wanted to compensate for not being able to protect his mother and probably Dante. Maybe he also wanted to protect humans as well. The other part would just be Vergil saying that line and it echoing in Nero's voice because Vergil wanted power.
Last part of the quote of becoming a demon, enduring exile and protecting "her". Nero obviously loves Kyrie and would do anything to protect her as it is stated in the quote. So if he becomes a demon blah blah blah; it's in the quote. But for some reason, I feel like it's also Vergil saying that. He knew the risks of gaining too much power and he accepted that fact of becoming a demon. He sort of did endure exile; never found his brother until a while, doesn't appear to be close to humanity or have any friends. The "her" is the curious part. It can't be Eva 'cause she's dead, so who's the her? I believe it's Nero's mother if Nero being Vergil's son is canon; it was confirmed, but there are some debates and plot holes. Vergil tried to be like Sparda so much that he sort of did. He emulated his hair style, was more willing to be a demon and accepting of his demonic heritage. Dante on the other hand was stated to only have inherited his father's rebellious spirit, while rejecting his demonic heritage, but then came in terms with it. Back to Vergil, he took it a bit further, he fell in love, like his father, to a human. Vergil found something worthwhile to protect and love. Although he may have clung to the belief that power can protect anything. Vergil is more of a son of Sparda than Dante will ever be.
Edit: After going through a rant, I completely ignored explaining why Vergil would pretend to be Dante. Short story: Vergil sees how much Dante is loved by everyone and tries to make sure no one finds out he's dead by pretending to be Dante. But someone, Lady, figures it's not the real Dante . . .