If I DM, then I can be assured that players will have spent upwards of several weeks or more developing their concepts, history, personality, and quirks. Since I personally stink at creating strategic dungeon crawls, darn near every one of my adventures is character driven through background or other pertinent "PC" factor.
As such, I have a very, VERY hard time killing off characters. When the player has developed such an emotional investment in their PC, and I've dumped a crudload of time into creating scenarios based on that PC, killing off the focus of a Biography doesn't really leave a good taste in your mouth.
I know its a serious DM flaw of mine, but I just can't seem to bring myself to kill off a PC that doesn't really deserve it.
Now, if they do something extremely stupid (that has NOTHING to do with roleplaying...some of my old players were silly bard-types, so they got away with some bizarre stuff), then chances are I've failed in creating a sufficient story that engrosses the player enough to be cautious with their PC. As such, I might kill off the player (as a result of their own actions) as a means to end the campaign.
Also, I don't typically use resurrection except as a plot device. During one campaign, the players converted some of their favorite 2nd Ed characters to 3rd Ed and acted as "Raised Champions" to battle a Gar'Udok like menace. It didn't last long, as I failed to scale the adventure appropriately (another DM shortcoming of mine) and the lvl 14 PCs stomped on everything.
So no, I don't normally kill off PCs.