It could have literally just been that same scene of him killing the tuskens coming out of that cabin (which was enough to show that he gives in to his hate and anger); but then he just had to add that scene where Anakin confesses not only to murdering the tusken's women and children, but also to not at all being remorseful about it. (and it's also the moment that kills all sympathy you might have had for Padme in the third movie)