I see they had the king and the duke astraddle of a rail- that is, I knowed it was the king and the duke, though they was all over tar and feathers, and didn't look like nothing in the world that was human- just looked like a couple of monstrous big soldier-plumes. Well, it made me sick to see it; and I was sorry for them poor pitiful rascals, it seemed like I couldn't ever feel any hardness against them any more in the world. It was a dreadful thing to see. Human beings can be awful cruel to one another.
What metaphor is Twain emphasizing in the above scene from Chapter 33?