I'm not sure why people hate unions exactly. Seems weird to have "freedom of assembly" but then hate those that practice it for better negotiating leverage.

This isn't about "freedom of assembly". I'm extremely pro freedom of assembly. "Unions = always good" is just as harmful as "Unions = always bad" as "Rich man evil" or "Rich man good".

Eh? I hate lots of people who I nonetheless want to have freedom of assembly. Like anti-vaxxers. I want them to be able to say antivax shit but I hate them. What exactly is weird about this?

Wanting to take away another groups freedom is not unheard of but it is anti-social.

Actually hating them probably means you have a lot of misplaced anger.