In Ender's Game, why was Peter considered too violent to commit genocide?

So I just watched Enders Game, great flick, but I'm a little confused. They talk in the movie about how Ender had two older siblings that had the same genius as Ender, but flunked out of the program because one was too passive, and the other (Peter) was way too aggressive. Ender was the balance between the two. At the end of the movie, Ender wins the war by completely annihilating the bugs home planet. How could Peter have been considered too violent to commit a total genocide of a species?

In the end Ender was left a broken little child because he was fooled into killing an entire species, and probably never would have done what he did if he'd known that the "simulation" were the real thing. Peter on the other hand, could have probably done the same job, knowingly eradicated the entire homeworld of their enemy, and come out at the end unaffected.

If the end result was the brutal slaughter of an entire race, then why was Peter considered "too aggressive" for the job?