Basically, Kyle in his anger might naturally gravitate toward a source of water to calm himself down. So he goes to the top of a waterfall, admiring the pretty view. He kicks out, in fury, at a rock or something. The mist from the waterfall has made the rocks (or grass, moss, etc.) under his feet slippery. Ergo, he falls and breaks bones. Unless he's got some trick where he can turn himself entirely into water and splash harmlessly down into the lagoon below and reform. (The pools under waterfalls are usually lagoons if they're not full-fledged lakes.)
