Why do our brains act in such a evil way sometimes that hurts other people?
MVR there are many hormones within us that motivate behaviours that served a purpose in previous (phylogenetic) environments that in our modern society may appear to be evil.

Oxytocin for example (the so called love hormone) spikes before conflict, and one of the biggest dopamine hits we can ever experience is by punishing someone.

Sapolsky 2017 - Behave: The Biology of Humans at Our Best and Worst