The cruelty is a means

There's been a bunch of people asserting that the cruelty is the point.  I can see where they're coming from, but, no, the cruelty is a means.

Any moral system is fundamentally arbitrary; you can get any result you want through selecting context.  (You can watch this happen in most of the press.)

If moral systems are widely used, this creates a social role for "selector of moral context"; it doesn't matter if it's as priest or a psychologist or a tabloid editor or a judge or a local gerontocracy.  Someone's social role involves picking the context for the moral judgement.

Once you've got the role, you want to keep it.  Keeping it requires that other people in your social system acknowledge that you have the power to declare the moral context.  Arguments over who has this power have historically tended violent; "a priesthood of all believers" is a statement that everyone (in context of its time, every male head of household) got to set their own moral context and make decisions on that basis.  There was a generation of war in consequence, because any existing system wants to keep existing and will not and cannot tolerate having the basis of its social power removed.  And this is a kind of power that must be used to be maintained; there's a constant competition for who really gets to say, socially.

That's what the cruelty is for; you use cruelty to demonstrate your power to set the moral context. (If it doesn't have a potential social cost to agree with it, it wouldn't be a demonstration of power.)  Facts explicitly and necessarily have nothing to do with it.  (This is why you have a class of very rich people proud of their innumeracy.)  People go along so they can reinforce their goodness, which is inherently what people say they are.

It's not the poor who go anti-vax; it's the upper class.  They go anti-vax because having to acknowledge intractable facts is equivalent to having to admit that good and bad are not what they say they are; that they do not have entire control of the moral system.  And in a moral system, either you have the power to declare the context -- to say what is good, and what is bad -- or you do not.

This is precisely why authoritarians insist that the problem is _saying bad things_, rather than doing bad things; saying bad things about people questions their authority, which is functionally the power to declare the context for moral judgement.  It's precisely why low-status authoritarians support the whole system; it's a known context in which they are good.  That's pretty much what it's "for" at the participant level; you've outsourced a lot of your insecurity management to the people deciding what the moral context is.  You get a lot of truly vehement responses, because any threat to this, even apparently entirely trivial threats, is equivalent to "I am going to hurt you until you admit you're a bad person".

Pretty much anyone engaged in that exercise of power started as a child in a moral system social context, and they aren't necessarily conscious of how it works and they either want to be good -- which requires a greater authority to convey -- or they're completely disinterested in anyone else's notions of good and bad, having defined "good" as "I get what I want".

The fix for this is not a better moral system (that great trap of the social left); any moral system inherently requires that selection of context because "good" and "bad" are preference statements and inherently contextual.  (Someone has to be doing the preferring.)

The fix is to argue measurable material outcomes and organize society around those; the difficulty is that most people would rather be good.  It's not a live-and-let-live circumstance.