I think have them discover knife early and domestication, then give them critters to slaughter makes them evil. I noticed if I don't get them the knife until the first IQ "level up" around 80 IQ, they tend to stay peaceful.
The IQ staying at 120 is a separate thing. I think if the humans build a church their IQ won't go above 120. I've had humans be peaceful and 200+ IQ.