I think I'd prefer to believe that the majority of people were basically decent and good-intentioned, even if it were shown to not be so (though in fact, the evidence bears it out), because believing so, and encouraging other people to believe so, might contribute to change.

Oh - and also, I believe I have free will, even though I suspect I probably don't, because believing (and behaving as though) I don't would be impossible because I would have no sense of the ' autonomous I' that was doing the believing.