If the Joker had members of the ACA hostage, for example Don Baker-- what would you say to convince the Joker he is being unethical-- given you have enough time to live? I was watching "The Dark Knight" and it occurred to be that the Joker is philosophical and has a different take on ethics. I think some would argue he is an ethical nihilist, but that is not the impression I get from some of his comments. He seems to have a clear, albeit destructive view of how people ought to live and what is Good to him is that people not "scheme" and follow the order set by the status quo. Christians like Craig have stated it would be hard for any one to live an amoral life, but to the Joker he seems to be at ease. I realize this is a fictional case, but people like this do exist, so the question remains. Given the fact that the Joker embodies the worldview he proscribes like Diogenes, the fact his values seem to conflict with that of society and does not care about others well-being, and trying to reason with him with appeals ad populum and to authority, especially mainstream philosophers and scientists is simply not going to work, what exactly could be said? So try not to think this is simply a waste of your time, but a chance to convince the Joker you have the right to live and he is unethical (whatever the hell that means).
I think there is no perfectly solid argument for being good instead of evil. The only one I could think of would be one of real world karma - which we have no reason to believe exists. IF it existed and we'd know, however, we could simply say that it would be self-service if the Joker would stop being the cause of suffering / death / destruction.