There is no slippery slope here. Harm was actual, demonstrated, and a direct result of the speech. No subjectivity in this case, and in fact a cast-iron proof that absolute free speech would lead to very nasty outcomes.
I'm not sure I understand your argument. It can't be proven either that someone didn't die of a heart attack right before the bullet hit them, but we draw a line at "really, really likely".
Not sure I can follow either. Bullets obviously cause injuries. Words almost never do. And if so, that's mostly up to the recipient and trivial to prevent.
Is a crime boss saying "kill him" to his henchmen not responsible too for the killing? Unless you disagree with that, we're only to argue where the line is, not that there is a line.
True, there are plenty of examples of 'harm' caused by calling people racists/nazis/granny-killers (during the SARS2-unpleasantness)/*-phobes/etcetera which have caused people their jobs and their social circles.
Is it illegal to label people with such epithets in the knowledge that it often leads to such outcomes? If not, why not?
