I don't think the particular issue is with deciding what
    hate is. Your example said that the AI would understand
    all of those as hate, and I think it would be correct.

What makes you so sure about that? What if the context is "I hate scientists...they keep proving me wrong". Is that a show of hatred or a tongue-in-cheek show of respect?

And then there's all the stuff that people say that gets misunderstood by other people.

And then there's all the gaming that's going to take place, where people realise they can silence their opponents by describing their opinions as being "hateful". Oh wait, that's happening already.