My next-door neighbor is fierce and unreasonable. What should I do with such a person?

It's unprofessional to fight violence with violence, but you can't quarrel with him. You can ask others for help to prove that you are right, so that your neighbor can't find your fault and he can't help it if he wants to attack. If he is really unreasonable, he can seek the help of the property or the police as appropriate.