Griefing: To create pain, discomfort, anger, or any ill related feeling toward another person in game.
This means when a NPC kills you in game, we can call it a griefer because it has made us earn debt.. Darn griefing NPCs!
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Okay, now that was sarcasm, but it seems that is how people look at it. To me presonally there is only few things I would take as griefing if done to me.
In UO there were teleporters. Some people loved to stay on them and not move just so lower level people would be stuck. No purpose, not for pVP reasons, just to be an a$$. Well to me that was annoying griefing.
I also think people that Rez-kill are griefers. For instance if we had people staged in the hospitals just to kill people that came through right after death. I guess its not griefing, but its lamer than lame and it would probably make me not want to be in a faction or PvPing type situation.