I have no doubt that Vergil being powerless to stop the death of his mother influenced him greatly to search for power. Yet, the fact that he said that without strength you cannot protect anything means that there is yet another person in Vergil's life that he desires to protect. If all he cared to protect is dead(Eva) then his desire to gain more power would be pointless. I mean, he was not trying to gain power to protect his bother Dante. In fact, he was perfectly willing to kill him (three times) in order to gain the other half of the amulet. So, to me, it looks like he was trying to gain power in order to protect someone else (so that what happened to his mother wouldn't happen again), along with his evil intentions of ruling the two worlds.
Another thing, I would love for the DMC story to turn from a solo hero, to a duo. Bring back the Dante/Vergil tag team like in DMC 3 (fav. of all DMC games). I want Vergil to come to the good side. Keep his calm, cool, no fear personality, but this time he too fights for humanity like his brother, just like his father Sparda(purple aura) would of wanted. The red just wouldn't be complete without the blue. It'd be awesome in DMC 5 and future games b/c you'd have an incredible two player experience. I think it be great to build on the characters of both brothers, since not only will they be constantly competing, but it would be the first close relationship the brothers would have since their mother Eva, especially for Vergil. Not only would this be a happy ending, but an explosive beginning in "Demon Destruction".