I'm running an evil campaign at the moment for D&D 4e, and recently I "accidentally" killed a player character.
Now said player character is a vampire, and he was level 4 with 44 hit points. Long story short, he killed a random NPC giving the player characters directions and I rolled a d6 in secret. 1-2 would be no repercussions, 3-4 some minor inconvenience, 5-6 the NPC would have a family member who wanted vengeance.
I rolled a 6.
So I created a level 6 paladin who was searching for his brother's murderer (in a later session) and who wielded an executioner's axe. They party encountered him in their travels, and he charged the vampire. Since their initiative was much better than his, all 4 of the party members went first.
Barely standing with 17 hit points, he bellowed a mighty roar - blessed his weapon and used Blood of the Mighty. I rolled a 19 - and since blessed weapon allows a crit on 18-20 on undead creatures, he would take 65 + 1d6 radiant damage - instantly killing him.
Damage was calculated as follows: 5[W] because executioner's axe has a high crit property + Str modifier (5) and 1d6 radiant damage.
Either way this would have put him to below -22 hp. But instead of his character dying I allowed him to live, but I need some sort of penalty to offset the generosity. I honestly did not want to kill his character for the second time.