There is a general cultural tendency to avoid hatred. A part of it has to do with the Christian tradition, from which much of Western culture is derived: "love thy enemy". Another part of it has to do with certain fanciful psychoanalytic theories which claim that hatred of any form is inherently harmful to one's psyche.

Do not buy into this. It is okay to hate those who have wronged you. It is a natural emotion. Your mind wants to defend itself. Your ego wants to defend itself. Your soul wants to defend itself. Hatred is one of the emotional tools it uses to accomplish this. Switching your mentality from "Why did she leave me? I thought she was perfect. What did I do wrong?" to "Fuck her, she's a stupid vapid slut" anyway is healthy. Yes, I said it. It's healthy. It's better to be enraged at her than to feel sorry for yourself.

There are studies on r/blackpillscience which demonstrate that bullies and sociopaths tend to be healthier physically and psychologically than average people. The hatred they feel towards others is a protective mechanism. Being a "good person" isn't necessarily conducive to good mental health. Evil people are often profoundly psychologically healthy, in the sense that their psyche promotes their social, biological and emotional fitness.

The converse is also true. Individuals who are bullied (or, in general, hurt by society in some form) will tend to feel better if they choose to stand up for themselves. They will feel better if they hate their bullies and choose to fight them, than they would if they just sat in a corner and cried about how unfair life is.

Hatred is a positive emotion, a healthy emotion, if directed towards those who deserve it.