I think that a conception of heroic morality (basically, whether or not to use TDT, or choosing between act and rule utilitarianism) may be at the heart of many of the choices to be cooperative/nice or not. Many people seem to assume that they should always play the hero, and those more virtuous ones who don't seem to think that you would never be able to play the hero.

As an example, consider assassinating Hitler. It's not clear how Hitler could reprise this -- he is already killing people who disagree with him, and he is a single tyrant while you are an invisible individual. This does not apply, however, if you are in equal factions, say Fascists and Communists.