Why are they so hostile though? The game makes it seem like their intelligence is to fault, but Fawkes proves that not only dumb mutants make up their armies.
Vault 87 super mutants are hostile because the transformation is so painful it drives them mad. Fakwes himself says he lost almost all of his intelligence and sanity after transformation, and it was only years of using the Vault's mainframe to learn things that he brought himself up to the level he is now.
Vault 87 FEV also makes super mutants increasingly more dumb as they age. While already stupid immediately after transformation, they get increasingly more so over time, unless they constantly work to avoid it like Fawkes did, making them even more unable to communicate and reason then they already were. Behemoths, the oldest of super mutants, are so stupid they basically only know how to make grunting noises.
Its also implied in the Fallout 3 game guide that either the FEV, or some natural human instinct to reproduce, drives them to kidnap people and make more of their kind.
So not only are they insane, and stupid, but they are driven almost solely by instinct.