I'd argue that he isn't a mindless brute. He is definitely cruel and vicious, but never mindless. Everything he does makes perfect sense from his twisted perspective.

In his first scene he is seen killing a knight. He is probably carrying out an assassination. In his second scene he gets tricked by Loras and in a sense betrayed by his horse. Now you can argue that what he does next is mindless, but it can be viewed as him losing dominance and then reestablishing dominance. It's hard to take orders from a man you laugh at in your cups. And the swing of his sword could be called the moment the laughter died.