How come some people have a chimp as a pet?

I've heard of a few people who had a chimp as a pet. there was even a chimp on a few shows, like Lizzie Mcguire! :) they are trained well it seems like chimps are cute sweet animals in general. but then i've heard of this woman who had a chimp who was friendly and sweet and she had the chimp for a long time and one day the chimp brutally attacked her and left her face in a very severe condition almost killing her. its just horrifying! why would a chimp react that way? and how come some people raised chimps when chimps can be this dangerous?