In a Forgotten Realms campaign a few years ago, the DM had done a masterful job of setting the party up. Bit by bit over a long time he managed to maneuver us into a spot where we could be falsely accused of conspiring against the city with the BBEG.
Once the net dropped, when we were all over the city taking care of our business, we each had a choice. The rest of the party decided to go off quietly, but my elven bladesinger wasn't about to yeild. When the city guard came to take him away, he hit them with a Sleep spell and headed for the hills (literally).
But THAT wasn't the Rubicon moment: The guy playing the ranger, seeing what I had done, decided that his character wasn't going to go along quietly anymore. So when the action came back around to him again, he challenged the guard, and then drew his sword (which they had allowed him to keep under his oath that he'd be peaceful), and KILLED THEM!!!!!
Bear in mind that this was a good aligned town and the town guard were innocent of any wrongdoing. They were simply doing their jobs. He killed over one of them, even after the rest of us were saying "You mean you do subdual damage, right???" "No," he said. "I'm aiming to kill."
Needless to say, this radically changed the campaign from that point forward. We ended up on trial for our lives, the ranger ended up being killed, and his head put on a spike. The rest of us were acquitted, but only because I was able to call on the priest of Tyr to adjudicate and cast a spell to compell us to tell the truth, so our innocence was proven.
It was actually a great two or three sessions, but it was one of those times when yo KNOW things will have to change going forward. To this day I can't believe that the ranger slaughtered the city guard.