The reason some people (formerly me) think there’s a “devil” is because we sometimes have terrible thoughts which seem out of character for us. We struggle against a part of ourselves.
It’s quite logical to think there must be some hidden entity that’s trying to persuade us to do terrible things. We certainly don’t want to believe it’s us doing it on our own. But chances are it’s just the tension between the need to survive and find meaning and our innate capacity for empathy. And in some cases, the brain itself has been damaged or altered in a way that causes the individual to have little to no control.
Not all evils are as evil as we think they are. Only a person with absolute knowledge could judge accurately everything that is morally right and wrong. Since no such person exists, absolute truth isn’t known absolutely by any mortal. As individuals, and societies, we piece it together the best we can.