Ambiguous morality systems can be fun in RPGs. After all if the "monsters" are attacking the village because the village founders chopped down the forest to make room for the village and surrounding farms rather than "we are bad bad evil bad monsters" it can throw a kink in the players plans to kill them all and take their stuff. Who the monster is is often merely a matter of perspective. It can be fun to try to mess with players in this way. Make them consider the ramifications of their actions. Of course they may just go with the "civilized" races are superior tack and kill the monsters anyways.