I don't think violence is the answer. A better statement would be "when they go low, we go lower". That would exclude violence from the equation and be a more accurate tale of events that Holder is trying to get across. As many of you pointed out, it's a statement that shows that being the nice guy only gets you so far, to quote one person "Trying to play nice when the other side deliberately refuses to behave in a decent manner, is in my opinion a losing strategy."
So to win, we must go lower than the other side, which would be, in Holder's eyes, to kick them. We cannot go high, or it is a "losing strategy". But advocating for violence is wrong, especially in a leadership role. I don't think Holder should have to resign or anything of that nature, as that would be extreme, but I do think advocating for violence is wrong.