"It is better to not have any resentment or take revenge.
Such feelings are against the law of nature.
Believe in Karma.
People get what they deserve as they go through life."
- Gu Family Book
I almost never learned this lesson. That standing up for yourself and being mean to others were not the same thing. But when you are not insulting someone, or hurting someone, or purposefully meaning to be a jerk to someone, you are most likely standing up for yourself. Enough is enough, and no matter how much nonsense you've been taking, there is a point where someone crosses the line.
This is where you have to be careful. If you're angry, you can say vicious things and easily cause harm to the person you are angry with, but if you are direct, honest and to the point, you are standing up for yourself and there's nothing wrong with this. But once you communicate this to someone, you should not seek revenge or even resent them. In fact, most people that harm others just for the sake of doing so are pitied instead of feared. I learned this today, when I had been tiptoeing around a person who purposefully spoke rudely and flat out ignored me. It only made the situation worse, and eventually, even after I asked a simple question nicely, the one snap they gave back crossed the line. I don't regret what I said, because I didn't insult them, or even bully them. I was direct and surprised even myself by the truth in my words.
So this is a lesson that must be remembered in order to go through life. When enough is enough, be direct, truthful and to the point. And doing so is not mean or rude or even harmful.