Yes, they can kill each other, but what you described sounds more like one of them defending its territory and family. If the beating is so severe that the wings hit the swan in the wrong way, it could do internal bleeding, neurological or other damage which could eventually kill the swan. The climbing on top is a behavior to signify submission that the weaker swan is owned by the dominate swan. Later, if the two boys survive and the submissive swan gets a second wind, becomes stronger, etc., then this fight will start all over again and the submissive swan may then become the dominate swan. This is unfortunately nature and how things are settled. If the submissive swan was able to flee and left the immediate area, it probably is well and will not come back anytime soon. If you were to disrupt the fight, usually, the submissive male would probably thank you and still leave the area and not return. The Regal Swan