No, there is no set recommendation that you must have a pair of swans as male swans generally stay on their own if something happens to their mate. Female swans will try to re-pair if something occurs to their mate, but again, this is an individual swan attribute and there is no guarantee that the swans will get along with another swan. This is especially true of swans of the same gender as once something happens to one swan, there is no way to introduce another swan of the same gender as this may cause serious injury or even death to one or both swans. Opposite gendered swans still may not be accepted.

As far as your swans, it is the onset of mating season (although early) and male swans' hormone levels will increase causing them to become defensive of their territory. Since both of your swans were doing well until recently, this poses several questions as to what might be occurring.

1. Has there been any change in or around the swan's habitat in recent days, i.e., mowing, construction, etc. This may cause swans to attack swans that they are usually familiar and friendly because they feel that their portion of the habitat is threatened.

2. Could the swan that was attacked been injured or ill? In nature, if something is wrong with a member, the flock/other members will chase the swan from the area. The last thing the flock wants is to have a disease spread to the flock or an injured or sick swan attracting predators to the area.

If none of the above is occurring, then it just may be that the swans were getting on each other's nerves caused by the increased hormone levels. It is not likely that the dominant swan killed the other swan as they may fight, but not likely to kill another swan that it has been sharing a habitat for so long.
More than likely the swan may have gotten tired of being chased and beaten and walked to another area to avoid the dominant swan. Unfortunately, this walking may leave the swan vulnerable to predators. If this is nothing more than a brotherly spat, the other swan should reappear as long as it has not gotten lost, is injured or ill or attacked by a predator. Please let us know how this situation progresses. The Regal Swan