First off- Don't kill your Players. I know they are a dime a dozen, but please- don't. If you want to kill someone, kill their characters.
Personally I am not much for just murdering off PCs (I prefer to let them do it themselves), because I find that Players aren't really all that interested in spending their gaming day making up new characters. They come to Play and have fun.
Also hard thought back grounds and such go by the road side when you whole sale slaughter PCs for no reason besides a desire to kill them off.
However if you need suggestions-
- Big battle scene. Much needed healing. Reverse the Positive Energy for Negitive Energy, so that healing spells become inflict.
- PCs are to rescue some prisoners. Make it tough, them sneaking into a camp, dungeon, tower, castle, whatever, then they flee. The Commoners are quite thankful. First night in camp the Commoners slip away, kill the guard with their Sneak Attack or death strike (Monk/Rogue/Assassins), then slip poison into the PCs water skins (just in case they are bested by the sleeping PC), then take out the rest of the PCs in their sleep. Should the PCs jump to their feet, and kill or drive off the Commoners then the poison will get them the following day.
- Think 2 Purple Worms
- A huge promise of richs at the end of the adventure, then have one of the traps be a Plane Shift to somewhere nasty.
- Grab the character sheets to start the BBQ (though your Players might get the hint that they are losing their characters)
- Oh, teleport them to the center of the ocean (five feet above). Two thousand miles from no where.