History provides us with numerous examples of people who were convinced that they were doing the right thing and committed terrible crimes because of it. Keep in mind that no one thinks of themselves as a villain, and few make decisions they think are wrong. A person may dislike their choice, but they will stand by it because, even in the worst circumstances, they believe it was the best available option to them at the time.
On its own, being a good person is no guarantee that you will act well, which brings us to the one protection we have against demagogues, tricksters and the madness of the crowds, and our surest guide through the uncertain shoals of life: clear and reasoned thinking. Logic will never fail you, unless you are unaware of—or deliberately ignore—the consequences of your deeds.