If you’re on those forums, maybe you should ask them.
While keeping in mind that anytime actual ‘hatred’ gets involved, any intellectual rationale is probably not the full story. E.g. I don’t even ‘hate’ people who practice abortion – I weep for them, grieve, and desire them to turn their lives around out of love for them and others – but I don’t hate them.
My most charitable guess as to why a certain type of Protestant might express a certain amount of… let’s say, froth, haha… towards Catholics, is that I think there’s a
certain type of Protestant (not all by any stretch, mind you, but some)… who believe that the Catholic Church is the ‘Whore of Babylon’, from the book of Revelation. (So it’s not just thinking we worship statues, worship Mary, all that other stuff they think we do that we don’t do – they think the Church was corrupted early on in history, and people who stay with her are being fooled by the devil.) And they’re deeply concerned that the Catholic Church (which they see as pagan-syncretist) is leading many people to hell.
Or at least, that’s what they say they believe. I can’t speak for them; I’d suggest asking someone for their reasons, if you’re curious what they are (but be ready to read and understand the Catholic responses, and understand how what they believe about the Church is a misunderstanding).
You may also end up getting more personal, emotional responses. Like maybe someone went to Catholic school and their teachers were bad representatives of the faith. I’ve met ex-Catholics who just say that in their experience, Catholics where they grew up were apathetic about their faith, whereas Protestants were enthusiastic and provided a good community for them to be a part of. Maybe they suffered abuse of some kind, know someone who did, or have just been scandalized by stories of abuse happening to strangers. It may not be theologically-based, is what I’m saying. But then again, maybe it is. I’d leave everyone to give their own reasons, and have an individual conversation with each person.
Ultimately for me I became (and remain) Catholic because truth is truth, and Jesus is the Way, the Truth, and the Life… and Jesus founded the Catholic Church, and remains with her, and so this is where I find him, receive Communion with Him, and will stay with Him.