This is nature. It is better to kill something that may have something wrong with it than let it suffer. In addition, if the cygnet is ill or injured, predators will get it and possibly follow it to the rest of the brood. Better to sacrifice one than the whole brood. Swans also do not want to risk the cygnet falling into the hands of humans. We have seen this behavior specifically to prevent a human from getting to the cygnet. This is the reason that if intervention is going to occur, it should be done swiftly or the cygnet may be killed before a human can get to the cygnet. The Regal Swan