This is nesting/mating season so the level of sex hormones is high and will take a couple of weeks to return to normal. In the meantime, this increased testosterone level will cause over protection of family and habitat resulting in fights. If the cygnet got into the other male's habitat or got in between the fight, it could have been inadvertently seriously injured resulting in death or outright killed.

We have seen infanticide by male swans when something is wrong with a cygnet, injury or illness. In nature, sick or injured members will be abandoned or removed by the members of the flock/herd so that disease cannot be spread to the entire flock/ herd or that predators are not lead to the rest of the family or
flock/herd by the affected member. So, yes the cygnet may have been killed, but more than likely caught in the fight and seriously injured. The Regal Swan