Hate speech is the difference between "I disagree with your opinion" and "You should be killed for saying this". The latter is hate speech for which its explicit goal is to exclude the victim from the debate by inciting hate on them. When done by multiple individuals, you get harassment and the very important risk of harming the victim (directly or indirectly, morally or physically).

Let's say there's a group called "We Will Kill Them All", you might say that this is just words, but people live by words (politics is literally just that) and hate speech directly and explicitly encourages people to take an act on the victim and harm them. You might say that any individual could do that, but do not underestimate the power of a group binding people in hate, because this is one of the strongest bonds you can make (We v.s. Them) and peer pressure makes people ready to do much more horrible things than they'd do by themselves. One such example is the NSDAP party, many of their members wouldn't have been against the Jews if they had never interacted with this party. So do not underestimate the power of hate speech.

To go back to the Facebook case here, the problem isn't the concept of "hate speech", the problem is Facebook's bad enforcement of it cuz legally there's just nothing against this.