Umm, if he wanted Sparda's power to emulate him, then he is going to use it to kill large number of demons and help humanity either as ultimate goal or lucky side-effect.
Sparda was savior of humanity, his power is either there for killing and/or saving, you can't say Vergil wants to emulate him to prove that he doesn't want to save humanity.
No, I mean emulate him in the sense of being the kind of being (yeah excuse the repetition) he was. As in, an incredibly powerful pure demon. Emulate Sparda in what he WAS, not what he DID, that's what I meant. Guess I couldn't get that across properly.