OK, I’ll explain this to you. Using the word as an insult means that it’s a bad thing to be. For example, the word sinister refers to left-handed people (though so far removed that most don’t know this). It was used to say something is evil, and in turn to call people who are left-handed evil. It was insulting to left-handed people by association.
If, for example, I were to use blonde to refer negatively to someone, it’d imply that it’s wrong to be blonde and blondes should be ashamed of who they are, right? Consider how you’d feel if we used a part of who you are as an insult. You wouldn’t enjoy seeing it used probably, right? Hopefully you’d have some empathy and stop, but maybe that’s too much to ask for.
Acting like you’re actually the one being oppressed is honestly hilarious while you insult a group of people.