The definition of Killer DM is somewhat unclear - I personally consider it to be a DM who racks up a high body count in his or her sessions, either because of too tough encounters or stupid players.
Most of my players consider me to be a Killer DM (I have over 50 PC deaths in about 150-200 sessions since 3E was released), but they still play with me because smart players do stay alive, and the games are fun. Consider me "Tough but fair"... mostly. In the few cases where I get an encounter badly wrong, divine intervention will save the party.
Certainly there are the malicious Killer DMs who just want to 'get' your characters, and design modules like "Tomb of Horrors".
However, for the purposes of this thread, a Killer DM is one who attains a high body count - for whatever reason.
Cheers!