Okay, but "can be speculative and messy" doesn't suddenly mean we can't make any judgments at all. Here the relationship seems quite clear. Why can we not act here, just because a theoretical example exists where it might be more complicated?

Let's make it easier and less messy. Supposed there's a policy that you know for certain would harm me and certain other people, but would not produce any other effects. Let's say there's a law that says I must pay a special tax that will be distributed among ice cream makers of the city. Ice cream is no Vietnam war, right? Nothing that awful. And if I don't like ice cream, such law is obviously not good for me. It's not entirely fictional examples - we have hundreds, if not thousands, redistributive laws on the books. They hurt some people, arguably help others. Some people claim these are positive-sum laws, some thing zero-sum or negative sum. But whatever it is, somebody is getting hurt - and not just by reading some unpleasant speech.

Should people be banned from advocating for or against such policies on Facebook? I mean, if one advocates for, it would hurt one set of people. If one advocated against, it would hurt another set of people. If you say we must raise taxes, you will be repugnant to Libertarians. If you say we must lower taxes, you will be repugnant to welfare state proponents. Should we just limit ourselves to posting cat pictures on Facebook? Nobody would disagree with cats, right? Right?

There's nothing 'speculative and messy' about the harm Nazis did and do.

We went to war over it.