only a few then did, and some of then were forced anyway.
Just like the Draenei who also drank the fel kool-aid anyway there and became sargerei, i know its funny to just point at the orcs but try to look the whole picture.
Of course humans are, thye slaughtr the whole easter kingdoms, draeneis are, mind you, 2/3 of their race joined feely the legion and slaughter countless planetsEveryone's capable of slaughter, but nobody is as eager as the Orcs among the playable factions.