The worst player that I have come across was a guy who thought it was a good idea to animate Zombies to use as bodyguards when the party was stuck in Ravenloft.
The worst I've seen was in the Night Below campaign, by Carl Sargent, where the party was given hospitality by the svirfneblin, a potential ally, and one PC robbed the graves in their crypt. He didn't trust the gnomes, and he thought they might be evil at the time. The party had to go the rest of the campaign without the gnomes' help, which could have made a pretty big impact but they pulled it out anyway.