There could be any number of scenarios. If the swans began to nest and were disturbed by boaters, humans or even shooting/hunting in the area, they could have looked elsewhere to nest. We would not put anything past the wildlife entities in various states trying to kill as many swans a possible for fear that they may also be stopped from killing them in the near future. So, yes the possibility of someone killing them would be plausible. Hopefully, the swans found another quieter pond to nest and you will see them again in the near future. The Regal Swan