I'm going to call an annoying guy a brainless monkey.
Why would you call him that?
He doesn't have a brain, he acts like a monkey.
Just because you find someone annoying isn't a reason to insult them like that. Comparing someone to an animal dehumanizes them and will hurt them a lot. Insulting people will just make people dislike you and not want to spend time with you.
You don't understand, if I called him a pig then it would still be bad. He was mentally impaired, a pig is better than him.
You shouldn't be comparing anyone to an animal, period. Saying stuff like that is really hurtful.