I may have posted these in one of the past threads - forgive me my repetition if I have. 
I almost always play good characters. Really good characters - paladins and the like. I just enjoy being knightly and heroic and sincerely good. However, sometimes, I get the urge to play really bad characters, if only because it throws my fellow gamers for a real loop, and reinvigorates my gaming mojo.
I have two such evil characters to share, with an amusing connection between them:
The first was in a one shot All Flesh Must Be Eaten game. The idea behind the game was “What if characters roughly equivalent to the ones in Mallrats got trapped in the mall during a zombie outbreak?” I ended up getting the “comic book expert” character, a la Brodie Bruce. My friend T? The manager of “Fashionable Male”.
Now, before the start of the game, I went over my plans for my character in depth with the DM - that I saw Brodie as an intensely selfish, if charming guy, who absolutely is out for himself above all others (as demonstrated repeatedly by his actions in the movie) and that he is almost supernaturally impulsive. The DM agreed.
Game starts, and during the character establishment period (read: no zombies), me and T’s character were predictably at each others throats. Then, yay, zombies! We and the other players all hole up in the comic shop, and being the foremost expert on all things zombie (yay horror comics), the group agrees to follow my lead in regards to plans to secure the mall. We ended up splitting into three groups of two, and of course the “random” straws had me and T as one of the teams.
Our first objective was weaponry. We hit the mall’s wall-hanger swords and cutlery store, and for objective two, were to check the mall theater’s exits to be sure they were secure. During this part of the adventure, I was totally agreeable, and “Brodie” and “Fashionable Male” seemed to actually be bonding in the crisis. Finally, we got to the theater, which was completely dark, and completely away from all the other characters. I offer to take the rear guard (so far, all the zombies had attacked from the direction of the food court), and offered T point. He took it. As we worked our way down the aisles, I asked the DM via note if T was paying attention to me. He made a check, it failed.
I then decapitated T’s character.
The whole damned game went silent - the idle chatter, the chuckles, the cheeto bag crinkling…all of it. T stared at me in shock. I looked at the group, and said:
“What - you honestly think Brodie Bruce, given a day of pure anarchy, without restrictions, honestly wouldn’t kill the douche from Fashionable Male?”
After a few moments, everyone laughed (even T), agreed that it was totally in character, and the action stood. Alas, poor Brodie was so caught up in his victory that he didn’t hear the zombie crawling down the aisle…
T and I then got to play a bumbling pair of drug dealers caught in another part of the mall. 
The second evil character story takes place in a 3.0 Forgotten Realms campaign. My DM was usually very opposed to evil characters, but allowed me to roll one when I presented a very detailed synopsis of his character, and assured him that I would not do anything evil against the party unless he was forewarned and able to plan for it. Alas, T (yes, the same T as above) heard I was playing an evil character, and decided he wanted to play one too, and our DM (whom I love, but was a doormat about such things) decided it was only fair.
Alas, the play style between us was so different. I was the epitome of Neutral Evil - I had no moral compulsions in any direction, but was not going to cause chaos for the sake of chaos - I had plans and schemes and working with the party served those schemes. T also played a neutral evil character, though I beleive it was more stupid evil - picking random fights, stealing anything not bolted down, and in general, much more open about his dickishness.
It was getting on everyone’s nerves, but there was nothing we could do against it at that time. Finally, though, we came to a combat sequence where hald the party (the goodies) were trapped across a stream of lava horribly outmatched, and the two of us on the dark side were on the other side of the room, with no enemies, after the bridge we’d crossed collapsed. The party bard threw across a rope, which T grabbed…and then he tried to pull the bard into the lava. He failed, but managed to get the rope burned up instead. More enemies started pouring into the combat, the rest of the party doomed, and T just laughing.
I passed a note to the DM - “What would it take to coup de grace T?”
The DM wrote back - “He’s distracted, but not helpless…but I’ll allow it on a natural 20 - otherwise, you’re in combat with him.”
I rolled. Natural 20.
My dagger pierced the base of his skull. The room went quiet. I coughed, said “What? He was in the way of my plans.” I grabbed the rope he had from his pack, and threw it across to the bard. The party lived, minus one stupid evil thief. The DM let T roll a new character, with the rule that he was not allowed to play evil ever again.
And T never again let his characters get in front of mine. 