Mine is being a killer DM. Sort of. Rather, my players feel that I'm happy when I kill a PC, and upset when I don't. For what it's worth, this isn't true, but rather, I want the boss characters in an adventure to be foes that are just barely overcome by a herculean effort on the PC's part; that sometimes gets misinterpreted that I want to kill the party with an unbeatable NPC.
The other thing is that when the PC's fight, I let them know it isn't quiet...and that attracts neighboring monsters. As such, they think I try to kill them since they invariably end up having a large number of foes in a single encounter.