As far as seriousness goes in a game, it really is a matter of one rule: is it helping your goal or hurting it.
If your playstyle or situation means that getting "shoot you in the face then T-bag you while going Xbox Live about their mother" results in a better chance at catching scum (i.e. the legendary "break your target" concept) then go for it. The only catch is making sure that you don't outright scare the person away from the game wholesale. No amount of "grow some skin" or "it's a game" is enough explanation of a lost player. Besides, part of good scumhunting is to customize your attack for your target. A player that breaks easy shouldn't get the full slamdown. Otherwise, you do what you need to.
The issue comes when you do an action that ISN'T pro-town. Whether it's going after someone because they annoy you, being so aggressive that the entire town stops hunting to deal with you, or being so gosh darn nice that everyone is having too much fun to hunt, the case is still the same: you aren't hunting and/or you aren't letting others hunt. Just stop doing it.
So I don't really think we need a 'be nice' rule so much. Games like this are supposed to have 'play to win' and 'don't disrupt the game' rules. Being rude isn't bad in itself. Forgetting the game in order to be rude (not playing to win) and being rude to the point where the game comes to a stop (disruption) is unacceptable.
It would be an odd game, though, that REQUIRES you to be 'nice' to your opponent. Sort of a 1984 style. That might be super plus plus good.