I didn't mean Vergil post fusion, I meant why does V possess so much heroic qualities when his original self never showed such a thing. I guess it falls into the line of thinking that since Vergil used the Yamato to separate his two halves, V who is the human half has all the "good" qualities, even if that treads on the toes of the franchise's message that demons can be good and that humans can be evil.Because Vergil's experiences and human emotions as V, as well as his conscience, carried over into his newly reformed self? I think it was pretty obvious.
Besides, it's not like Vergil did a complete 180 and now he's some compassionate, generous soul or anything. His demonic side is in there too. So while he does certainly seem to have lightened up a bit (for example his humor is not as cut and dry with that layer of threat to it like in DMC3, it sounds a bit more light hearted in nature, like his attitude in general), he also is the guy who refused to acknowledge Nero ("My son means nothing to me") until he proved himself as a warrior.
They showed he has changed a little, but not so much that it's a jarring transition from what he was before.
Anyways, that was more of a minor bugbear than anything else. What really feels jarring is how Vergil now has the highest body count of any antagonist if you put his crimes from 3 & 5 together and yet that kinda gets swept under the rug after Nero decides he won't let his father die.