And the thing is, the Joker could so very very easily given a reason why he can't be killed; he's a monkey paw and killing him just means some other poor bastard becomes the new Joker, like a memetic viral version of the Slayers from Buffy (if you ever interact with the Joker, you're a potential host when the time comes for the legacy to pass on). The kicker could be that they may have different interpretations of the role, so one might be a psychotic killer, another a caperer that don't deliberately kill while another specifically won't kill because it's funnier to leave his audiencevictims alive.