Some things are objectively hateful. "This class of people should not have rights" is objectively hateful. "This class of people is subhuman and we'd be better off if they died" is objectively hateful. "This event that killed dozens of people was good, actually" is objectively hateful. If we can't agree on things like those then we have no common ground. If we can agree on those then it's just a question of where the line is, rather than whether a line exists.

Just because it isn't a perfectly bright dividing line doesn't mean it's entirely subjective. It is also to the benefit of a side regularly spewing hate to also argue that it's subjective.

(And to be clear, none of those things short of imminent threats should ever be considered punishable by a government; that has far more negative outcomes than positive ones. But they absolutely should have consequences by individuals and groups who choose not to associate with such things.)