Words can get someone involuntarily committed to a mental hospital. Words can be used to take away rights. Words can affect national policy. Words were what Adolf Hitler used to send people to the concentration camps, and they're what Donald Trump is using to do the same thing today. Words are extraordinarily dangerous.
When we legitimise words that dehumanise the mentally ill, words like r*tard or n*rcissist, we give more power to fascists, because they can go on to use those words and people won't be offended. Ordinary people's offence is a defensive weapon that can be used to protect against the misuse of words. Ordinary people's offence is a valuable resource it makes sense to cultivate.
I want people to be more easily offended, so that they'll resist messages of hate spread by fascists. If people learn to be okay with hearing slurs casually thrown around on the street, words like f*ggot and n*gger, then things are going to get worse for the people those slurs describe.