idiots who like to train them to be aggressive.
I know some irresponsible dog owners train their dogs to be aggressive, but in my experience, I've had more problems resulting from people who can't control their dogs. There's nothing worse than a dominant dog with a submissive owner. And, I have no use at all for people who let their dogs run loose, and then are so surprised when something bad happens.
I had two female Pitbulls, litter-mates, and one was fine to take anywhere and the other couldn't be trusted at all with other dogs or cats. I didn't train the one to be aggressive, but I did put some effort into making sure she was never allowed to run loose. Eventually, we couldn't allow the two to be around even each other.