I'm going to take a swing at your argument from a different angle, so follow along for a second. How do you define an immigrant?
The reason why I bring this up is because I've often seen people embracing laws or actions targeting immigrants, but isn't this in effect the same slippery slope as attempting to define extremism when we talk about free speech? Especially as immigration law is often expanded over time, encroaching on the rights of 'citizen' (a term which is also eroded over time) as we define a secondary class of people of which your typical rights do not apply to.
If we want to play the definition game, the exact definition of the word 'immigrant' depends entirely on the ruling class, ergo makes laws affecting immigrants equally as likely to cause the slippery slope effect as laws that affect extremists.
Well, broadcasting your massacre of 20 people online would be one kind of 'extremism'.
So there's that.
Surely it can get tricky (and it will) but we have to do something. This is not new, we always have. Every nation has some kind of hate law, applied one way or another.