Hatred doesn't only return through external means, such as assassination or legal retribution. It actually returns at source. I don't know if it says it in the Qu'ran, but in the Bible, Ecclesiasticus 13:1 says "He who toucheth pitch shall be defiled." You let hatred fester in you, you fester yourself.

I never met anybody who was happy with their hatred. My Dad always said "What you are eventually shows up on your face" - you look at trump now, when he's not trying to charm the voters. That man is festering hard.