DroneRights

https://en.wikipedia.org/wiki/Pejorative

A pejorative word or phrase, slur, or derogatory term is a word or grammatical form expressing a negative or a disrespectful connotation, a low opinion, or a lack of respect toward someone or something.[1] It is also used to express criticism, hostility, or disregard.

https://www.merriam-webster.com/dictionary/slur

an insulting or disparaging remark or innuendo

https://dictionary.cambridge.org/dictionary/english/slur

a remark that criticizes someone and is likely to have a harmful effect on their reputation:
an offensive word used to insult someone because of their race, sexuality (= the fact of being sexually attracted to people of a particular gender), etc.:

https://www.dictionary.com/browse/slur

a disparaging remark or a slight:

The N word is specifically a slur referring to a mental disorder, and is thus an oppressive slur

https://en.wikipedia.org/wiki/Narcissistic_personality_disorder

Narcissistic personality disorder (NPD) is a mental disorder

https://www.mayoclinic.org/diseases-conditions/narcissistic-personality-disorder/symptoms-causes/syc-20366662

Narcissistic personality disorder is a mental health conditio

https://www.britannica.com/science/narcissism

Narcissism, pathological self-absorption, first identified as a mental disorder by the British essayist and physician Havelock Ellis in 1898.

It's not an opinion. Opinions are likes and dislikes, and subjectives with no evidence. This isn't one of those. There is a clear definition, and clear evidence that it meets the definition. It's a fact.

Of course I don't get to decide what's a slur. The people who use mental conditions as insults decide that.