Peter, from the Ender's Game
series, is actually a very complex character. The first book, the only one in which Peter appears, is told through the eyes of Ender, who cannot really be considered objective. It is fairly obvious that Orson Scott Card
did not intend Peter's character to be a simple one, and this is especially obvious when Valentine
cycles through the tortuous cycle of logic deciding whether to help Peter with his conquest.
Simply put, Peter may have had "The soul of a jackal" (as Colonel Graff put it,) but he channeled his destructive energy into channels to minimize the damage to other children. The admittedly twisted animal torture is hypothesized to be "a sacrifice that somehow stilled the dark gods that hunted for his soul." These 'dark gods' were something that each of the children in the Wiggins family combated. Peter explained himself to Val admirable; "I don't want to destroy anybody. And I don't want war, either. I want the world to hold together. Is that so bad?" Peter didn't hate anybody, but he "had to have control... It's the most important thing to" him.
The question that still has to be answered is: Was he telling the truth. The first hint we have is that Valentine believes him, but later we find a much better proof: He takes over, and then he builds. The only real proof, it seems, is to stand the test of time, and Peter Wiggin, Hegemon of Earth, certainly did more good than damage in that regard.
Whether or not Peter's motivation was just selfish, or also partially masochistic is left somewhat ambiguous, but the success of The Hive Queen and The Hegemon seems to indicate that his motives were justifiable after all.