Interestingly enough, most of the PC deaths in my campaigns have been at the hands of other PCs. I've actually had a TPK because of party infighting. Granted, I provided the scenario that caused the argument which ended in lead flying, but I didn't pull the trigger.

That said, I like my encounters to be difficult but manageable with proper planning and tactics, and I don't fudge. Thus, if the players don't approach a situation intelligently, it's entirely possible they'll die.

My first PC kill was actually in a home-brewed zombie apocalypse campaign adapted from D20 Modern. The game had the standard slow, stupid hordes, but also had some special infected, ala Left 4 Dead. I had a player who insisted on being exclusively a melee combat character, despite many party urgings to the contrary. This same player worked with me on a backstory in which his older brother had been bitten while defending him, and told the PC to run while he held them off as long as possible.

Naturally I had to use that somehow, so the brother wound up being a slightly more powerful version of the Hunter-type special infected (pounce, claws, speedy, and able to track by scent). After already being wounded from an earlier combat, he decided to try to take the thing on, solo, with a sword.

My description of the result was, "He's playing with your guts like a two-year-old plays with spaghetti."