If you find this horrifying (and I hope you do, because there can be no moral justification for celebrating murder), then I encourage you to really think about whether we would not be better off without such extremist language poisoning people's minds. We have to try to stop escalating, or the cycle is going to destroy our society.
Well, yes. People point this out regularly with mass shootings. Sometimes the shooters helpfully leave a list of all the violent rhetoric that inspired them. Anders Breivik claimed to be acting against an "existential threat". Those words get used a lot.