I totally missed this thread! What follows is probably the worst thing I've done to a fellow PC: murder.
Back in college I played in a brief mini-series D&D game with some friends of mine. The game was based around our characters essentially being a pirate crew, though not in the traditional sense. We were on a ship headed towards this island that was said to be under this powerful curse. The captain of the ship was this drow matron of some sort. My character was basically one of her thralls: a lawful evil human wizard with plans to become an assassin. The captain's "first mate", and other member of our group was a drow priestess who just loved to torment my character any chance she got. The third member of our party was a druid who mostly kept to himself.
During our journey to the island, we had some downtime for character interaction and getting to know the crew of the ship. There are a number of unremarkable lackeys, but one of them in particular ends up standing out. It's this goblin cook that I figured out was actually an assassin. He feigned stupid quite a bit to throw off other members of the crew, and it turned out that only I, the druid and possibly the captain were aware of this.
Throughout this whole trip, the drow priestess continues to harass, belittle, and at one point throw what sounds like a death threat at my character. By this time we had just arrived at the island where the captain, since I was basically her slave, charged me with going into town to pick up some magical items. I figured out that she'd given me double the amount of gold that I needed to buy the items, but that she would require some form of receipt to reconcile her spending. This is where I saw the opportunity to hatch a plan, get some revenge, and maybe even pre-emptively protect myself. I took the list of items to the various stores, purchased what was needed and ended up with 2400 gold left over. I then bought a scroll of minor image, casted it on the receipt to make it look like I had spent all 5000 of the gold on the items, and then casted nystul's undetectable aura over that so that the alteration couldn't be detected.
I pocketed the 2400 gold, presented the receipt and items to the captain, and then paid off the Goblin cook to kill the drow priestess. He tells me that it will happen that same night after everyone has gone to sleep. So I get ready to be there to witness the act, not willing to get my hands directly dirtied. The goblin shows up eventually and begins killing her right on the top deck when mostly everyone had gone to sleep. He doesn't finish the job as efficiently as I would've liked, however, and the druid wakes up to the sounds of the priestess crying out. It becomes clear that he is probably going to see this go down before we're able to hide the body, so to speed things up I step out of hiding to finish her off with my quarterstaff.
The druid catches the end result of this and reluctantly agrees to keep his mouth shut.
Thinking back, I probably didn't need to resort to killing another PC, and it was the only time I'd ever done so in a game. But I was so focused on roleplaying lawful evil that it just made sense at the time. Despite this, everyone (except the drow priestess player) had fun. The player and I are still on good terms and ended up dating.
