Partly because the fewer insane Judges, the better; especially the Judge associated with water. I haven't seen a spider since Surgeon died, and that was one of his associations -- think about what would happen if we lost Waterbearer.
But mostly because there was something I could do; he was suffering and I had a chance to help relieve it at no cost to myself. Not taking that opportunity would have been cruel, in a way, and Waterbearer's never done anything to us before today that would justify turning my back on him.