I personally, as GM, don't like to be the one that kills a character. I want to set up a situation where the situation-and/or-the-rules kills the character.

I've played in games with "the GM kills you" mechanics and it felt juvenile/arbitrary/abusive. Remember, this is a game where every players' character needs to "shine" and similarly needs to know they're being judged with fairness and compassion by the GM (compassion especially along the lines of "understand what I say my character does as something reasonable").