There was some guy from Animal Planet on the news last night. He said it's completely normal for a chimp to do this type of thing and that they usually will show there first sign of violence at about age 6. That's when they mature to where they don't do things just because you tell them to.By nature they don't want to be dominated.
They also had a couple on that had two chimps until about age 7 when one flipped out and they gave them both to a zoo.They also went on to remind people that these are wild animals and we not to be raised as pets and those people that think that if they " loved" the chimps enough that theirs will never become violent. When it's guaranteed that they will sooner or later.