Take an issue on which people were really split, take slavery. It wasn't just an intellectual debate, obviously - there was a huge amount of struggle involved - but insofar as there was an intellectual debate, it had a certain shared moral ground to it. In fact, the slave owners' arguments are not so simple to answer-some of them are valid, and have a lot of implications. They were taken very seriously by American workers in the late nineteenth century, for example.
For instance, the slave owners argued, "You take better care of a slave if you own it than if you rent it." Like, you take better care of your car if you own it than if you rent it, so you take better care of your worker if you own it than if you rent it - so slavery's benevolent and "free market" is morally atrocious. And the slave owners in fact said, "Look, we're a lot more benevolent than you guys with your capitalist wage-slave system." And if you look back at the literature by workers who organized into, say, the Knights of Labor and other working-class organizations of the late nineteenth century, you'll also see a strain running through their position which said: "We fought to end slavery, not to impose it" [i.e. the industrial wagelabor system became dominant after the Civil War]. So the point is, on all sides of debates like these, people understand that they have to appeal to the same basic moral principles, even if what they're doing is totally venal.
I mean, it's extremely rare even for an S.S. guard or a torturer to say, "I'm doing this because I like to be a son of a bitch." We all do bad things in our lives, and if you think back, it's very rare that you've said, "I'm doing this just because I feel like it" - people reinterpret things in order to fit them into a basic framework of moral values, which in fact we all share.