to first check who at an Israeli festival is a settler and who isn't before

This is what the RAF did with the kidnapping of the Landshut flight. Which was a problem as some Germans did select people for being Jewish/non Jewish. But yes, if you do stuff like that you have to discriminate in any case.

However what you do tell, too, is that killing the ravers (and taking them hostage), is okay in any case as corollary. Rojava was able to fight Isis more targeted than that. The PKK of old did target more focused, as was Mao's and Enlai's actions.