Insults a form of violence. Emotional violence that is. If I wanted to, I could rather easily insult you. Therefore I am capable of violence. However I did not default to it, because the situation does not call for it.

You could have replied in whatever way possible to my comment. I did not insult you at all. You however defaulted to insulting me. So unlike me you are not just capable of violence, you default to it.

That is the difference between being capable of and defaulting to. Hope you get it.