Not that I have a horse in this race, or even have had the pleasure of roleplaying with a great many of those listed, nor the to be victim, but I chose the Dark Lord of the One Sith.
My reasoning is that it is the truly one way to ensure the character doesn't come back. If the author of whoever does the killing goes away, there is less impetus to keep the character dead. Having the NPC Dark Lord do it ensures that the killer will always be around to act as a deterrent to character revivification.
That is just my two cents, I have no reason to mistrust Enigma's honor in saying the character will stay dead and I whole heartedly approve of using the death of a character to solve an IC beef. It should be good RP and I would very much enjoy reading it when it goes live.