I don't consider myself a "killer" DM, but the joke among my players is that new PC's aren't full-fledged members until they "join the ranks of the dead". The current campaign has been going on for about 2 years now, and 7 of the 8 players have had their PC die at least once.
Basically, I try to key my adventures so that it will be tough to survive if you do stupid things on a regular basis. I have had a couple encounters where a TPK was a distinct possibility, but the group has always managed to pull through. And if I key something overly tough, I try to ensure that the dead can be returned to life.
The most recent near TPK involved a 16th level Cleric BBEG, a 15th level Sorcerer BBEG, and 2 12th level fighter-type half-dragons mounted on greater drakes for cover and support. (The party rules-lawyer had an absolute fit, then decided I played them stupid when only 4 of 8 PC's died.) I feel that epic-type threads BEG to threaten complete havoc. Also, this particular group of PC's are an average of 14th level, and have a fair amount of renown in my campaign (due to aforementioned rules lawyer hiring several bard NPC's to publicise certain events invovling the party to promote a special tourney held by the local Duke - ALWAYS, as a DM, apply the Rule of Unintended Consequences.

) As a result, they have attracted the attention of some VERY powerful BBEG's. But they also have the services of a 17th level Cleric (NPC Ambassador) whom they have helped out a couple times.
I also try to take into account the experience level of the players (not the PC's). We have a core group of 6 who have been playing D&D since 1st Ed., NOT including me. Players like that are generally able to find ways to deal with much heavier challenges than new players would even dream of. Essentially, these guys prevent me from being a "killer" DM because they are too good as players. Some of the stuff they have pulled off with few deaths would have TPK'd most less experienced groups.