Because this is violence that is not acceptable to the ruling class. It is violence against the ruling class. The first grader is a child of the working class. That type of violence is what is "acceptable" to people like Charlie Kirk.

That's what his famous quote about accepting a number of gun deaths every year actually means. He was never including himself in that context.

These people tell you what they believe. You just need the lense of class conflict to view their words through.