Schizophrenia and manic depression fall into that category. Viewed in light of what happened early in life, an adult's erratic and sometimes destructive behavior is entirely reasonable. The problem is that, without a knowledge of the individual's past, you are left to guess as to answers. Some people live by the wolf pack theory. Wolves have an uncanny ability to detect congenital weaknesses in their prey and to attack weaker animals.
Some people are like wolves.
They join in attacking others they perceive to be weaker and less competent. Not pretty, but a fact, nonetheless. Love and gentleness are acquired qualities. Early environmental conditioning has much to do with whether a person grows up to become open, loving, and gentle or defensive, aggressive and harsh. Based on the amount of sheer awfulness in the world, it's not surprising that few adults come through unscathed.