I think you put it into words better than I ever could have. Very powerful scene indeed. And when you think of it Arthur has put his life/neck on the line several times before (ie. 1x11, 2x09, 4x02, 4x05) He has always proved willing to sacrifice himself before anyone else, but this one stands out because of the heightened emotions surrounding it. He also has seen and learned from the repercussions of Uther's own past mistakes and is willing to notice, acknowledge and right his own before it is too late. I thought Merlin standing at the well and hearing all the voices from the massacre was well done also. Seeing Merlin trying to process all the voices, hearing the little boys voice through it all, grabbing his head because it was overwhelming him.I can only imagine what it would be like to have the power to hear voices from the past and not be affected emotionally/physically by it. There have been a lot of scenes this series that have showcased both Colin and Bradley's growths as actors.