There is a fellow player who is in the group of Pathfinder players I find myself playing with. I only play with him because we have a mutual friend (the DM). He is always trying to attack people for no reason.
Once, there were two NPCs who knew where our main opponent was. We were questioning them when for no reason at all he began attacking them. This made it much harder to find the bad guy in the story, and they were part of a tribe, so anyone we encountered along the way who was also part of this clan would attack us.
Later in the day's journey, I would have my character physically force him not to attack NPCs. A problem with that was that the chances of my character one-shotting said character was high.
I know that some time in the future I will play with him again, and want to know how to deal with him. If anyone would like further details just comment, and I will answer your questions.