If you start out with the assumption that someone is a self-serving and horrible person, everything they do or don’t do–no matter how innocent–will appear to be self-serving and horrible. If they, for instance, go out to a movie or to a concert, it’s because they’re frivolous and selfish. If they fail to share that one article on Facebook, it’s because they’re a cowardly hypocrite. If they donate time or money to a good cause, it’s because they’re angling for something.
Even the most charitable acts can start to seem monstrous. This is how you get people who think Mother Teresa did everything she did out of some perverse desire for recognition.
I’m not saying any of us are Mother Teresas, and I’m absolutely not saying that objectively wrong acts can be forgiven because the doer had good intentions. All I’m saying is that it doesn’t hurt to give people the benefit of the doubt. It’s better to err on the side of thinking people are better than they are than to spend your whole life despising everyone for the ulterior motives you’ve assigned them in your head, which may be miles from the truth.