People get mad if you use a slur because it's a slur, an insult.

Also you try to use "biologically" in your question to pretend biology has something to to with it. But no one uses "biologically like that. Let's take it out. Why do people that "look black" dislike it when people who "look white" use the n-word? Because the origin of the use as a slur dates back to a time where people that "look white" actually deemed people who "look black" as a "biologically different race" (and again, despite a use of the term biologically, no actual biology behind it) and withheld equal rights on that basis. So today, without an added cultural context, the word is perceived as a slur. That's the current default state.