I don't think we should be imprisoning the mentally ill (that's more of a conservative thing) - I think we should be giving them access to treatment. From there, the bulk of the conservative brain damage should resolve itself.
Imprisoning those inciting violence against minorities based on lies and attempting to overthrow the government based on more lies seems like a responsible step in protecting Americans and American democracy though.
It's hyperbole. They were mocking you for using "mentally disabled" as an insult. You're the one talking like a conservative right now, using disability as a pejorative.
It's not true when Conservatives say "Liberalism is a mental disorder", just as it's not true when Liberals say "Conservatism is a mental disorder". The only thing that's true in that context is that supporters of both major parties have no problem throwing people with mental disabilities under-the-bus over politics.
Republicans are stupid, they are ignorant, they are hateful; but none of those things constitutes a disability. If they did, I would have compassion like I would for any other disabled person.
I think you're missing the point of the original comment. Describing modern conservatism as a disability at once absolves conservatives of their responsibility not to hold abhorrent views, by classing it as a characteristic they have no control over, and lumps people with disabilities into a group that, in a non-insignificant number of cases, wants them dead or sterilised.
That's why the commenter was upset with you; not because you've criticised eugenics, but because you've been massively insensitive, and when someone pointed out that insensitivity you became defensive and attacked them in return.