That is equally vague. What constitutes harm?

Any action or inaction that physically, financially, or mentally damages another human being through malice or negligence.

who defines what harm is?

In a democracy? The people.

Belief happens in the mind. We don’t want to go down the road or thought crimes.

If you believe hurting a group of people, for any reason, is righteous you're more likely to commit crimes against that group. If we're going to start talking about slippery slopes, let's talk about the slippery slope of allowing hate to take root and spread in any society and how that's turned out in the past.