The following material is flagged Red Level. It deals with the blogger's original ideas, personal beliefs, and delusions; and might not be believed by any expert in any field anywhere.
Every atrocity committed was committed by one who believed that they were in the right. Every hurt inflicted was inflicted by one who believed that the harm was deserved, or that the good of their own group (whether that group was a race, a nation, a species, or a planet) demanded that the hurt be inflicted. How does one determine whether one's intentions are good or bad?
Correction upon further reflection: Some harm is inflicted by people who aren't thinking, or who simply don't care. But accident is inevitable, lapses in self-control are by definition beyond control, and psychopaths are a very small minority of the population. Let's constrain this discussion to deliberate and intentional harm inflicted by non-psychopaths.
(Oh, and by the way: I do know what I want to write next: a formal version of the Devil's Proof. Unless something more interesting comes up.)