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.

Nah, none of those. All instances of harm require unnecessary action taken by choice. Words can be disregarded. Acting on words is the actor's choice.

When we legitimise words that dehumanise the mentally ill

They're not doing that. Moreover, using such words alone doesn't do what you claim. There are a number of steps between a word you find offensive & adverse action: that argument is a slippery slope. Unless the words incite imminent action, people have an unbounded amount of time to think & arrive to a decision before taking action. Any amount of discussion can occur during that time to influence & inform decisions. Rather than an overgeneralized attack on using a word, a more focused & coherent argument to support human rights could be raised.

Over relying on offense & emotion to steer their judgement discounts people's capacity to reason & infantilizes them, which is condescending. Offense & emotion are not reliable guides of judgement. Speculation that it would promote better outcomes is not a valid argument. That such an approach would work better than reason is poorly supported. We could at least as plausibly appeal to reason rather than to offended emotion with the bonus of not irrationally overgeneralizing.

People can interpret context to draw distinctions & you're overgeneralizing. The overgeneralization underpinning your offended opinion isn't a valid argument. Neither is the speculation offered to support it. Telling people their words mean something they do not, disrespecting their moral agency & ability think, & insulting their intelligence to discern meaning is unpersuasive. Promoting a rational argument more specifically supporting the outcomes you favor would be more persuasive.