House rule: No discussion of comparative ethics at the table, no between-player discussion on the matter at all. All such debates are strictly player-to-DM and only on their character, never anyone elses.

OTherwise it invariably blows up into one of two scenarios, someone plays a paladin and proceeds to use detect evil on everyone they meet and kill them if evil (seriously depopulating NPC villages) or someone pulls a 'well, my culture believes in mass human sacrifice and thinks its good' which last time involved a 15th level character 'getting religion' and wander around with a club to knock out people and sacrifice them... which was actually fine, I was DMing and I encourage rampant forays into relative evil so I was given him a +1 sacred bonus to the attribute of his choice for every 100 people he scarificed to his self-created deity 'Quencil-Nyquil, God of MAss Human Sacrifice'.

You can always find a game friendly solution to almost any problem, you just have to make sure everyone knows that major disputes between players are no-nos.