Human nature. Those who go through life accepting what is fed to them as "positive" and "negative" by whatever section of society is most immediately visible to them tend to look down on those who hold differing views. If those differing views are presented as a violent rejection, the conditioned return response is violent rejection of the person holding those views. Even if such things are initially presented as a joke, if those involved don't study their own motives, the joke tends to be taken too far.