I was always taught that the first person in a discussion to begin name-calling, raising the voice and begin shaking his or her fist in anger was the first person who had run out of ideas.
I never met Charlie Kirk or attended his speeches. However, from the few videos I've seen, he would speak calmly with anyone and listen – actually listen – to others' viewpoints even when he disagreed. That is an American tradition, that we are allowed to speak our minds and we will defend our political opponent's right to do that, also.
Charlie Kirk apparently believed in that tradition; his assailant did not. The entire country should grieve that, once again, freedom of speech has led to an attempt to silence it.