My theory about Nero is that somehow his arm has become possessed by the spirit of Vergil. Some reasons i believe this are: He was able to revive yamato, Vergil's sword. When Dante confronts him about getting yamato back he says that the sword "has to stay in the family" and then ends up letting nero use the sword, and eventually at the end letting him keep the sword. As many other people have said the phrases he uses are almost literally "Ctrl + C, Ctrl + V'd" from vergil in DMC3:SE and his general demeanor is very similar to that of vergil. I have not thought about this from every angle so feel free to call me an idiot and tell me why my theory is completely wrong but this is what think of the situation.