Eltsac,
You just look your buddies in the eye and say...
"I am only killing you because I love you."
Or.
"insert name here, I love you... but."
That is our group's favorite table top explaination for killing someone else's character. It's always funny when the Storyguide calls you to let you know that you miiiiiiiiiiight want to come up with a new character for next ame.
I dunno if I like the idea personally, of people playing NPC's on a Mud. There is just too much stuff that has to be communicated and generally, it's easy to do that at a table top. You can use your voice and just tell someone. Generally, I don't think the IMM's have enough planning/time before they start running things to make sure they include that condition that if they NEED someone to play an NPC they can. That is a lot of typing even for simple things, in general. (Shove the code lingistics in the back.) I mean, the sheer amount of communcation that has to be present in text form.. *whistles*
Of course, that is unless the command is, "Kill them." To which, why arent' they just using the NPC themselves and set it on "Kill everything in sight." Mode?