Their rage against Rowling comes from their absurdly obvious misogyny. They have to keep the women in line. As far as they are concerned, women are meant to serve the interests of men - in this instance, trans-identified men, but in other instances for example they might "cancel" women who refuse to support prostitution (which serves the interest of male johns, pimps and traffickers) - and women who refuse to do so
must be made public example of to keep the rest in line. JKR is a high profile woman who is openly refusing to cooperate with their agenda, so she's being made example of; it's meant as a message to all other liberal women: Look what we can do to one of the most wealthy, powerful women in the world, we can make her
persona non grata to the entire world, lose opportunities for her, make her word a dirty name, send bomb threats to her without consequence, we can threaten to rape and murder her and enjoy it and the police won't even investigate us - we can do so much more to you and
no one will help you, so stick to the party line. It's a powerful incentive for women to keep their mouths shut.
(And they
do do those things to less powerful women. Women all over the Western world have lost their jobs, their livelihoods, sometimes their homes, been threatened with rape and murder, etc. by trans activists for failure to tow the party line. But those women were not high profile enough to be known all over the world - JKR is, so it's useful to them to make example of her.)
And liberals say conservatives are misogynistic. Whoo.

(I mean, they are, just in different ways. Politics sucks.)