Good and evil.

His mother was killed, which means this person is a murderer, therefore, he will kill again, therefore he is not worthy of life, and since the court acquitted him, I will have to finish what was started.

If all people understood that revenge breeds even worse revenge, then there would be no avengers as such. I think we wouldn't even know such a word, but that's how it historically happened. This is how evil is committed, which a person may consider good.