Unfortunately this is the “ugly” side of nature. It’s unpleasant that some animals have to die for others to survive but that’s just the natural life cycle.
Predators kill their prey for food but sometimes animals can also kill for territory or mating rights. Some of this behaviour can be hard to understand, given that the perceived benefits seem so little, but keep in mind that everything in nature is subtly interconnected. For example, an animal might kill another so that the latter doesn’t use up the natural resources available in that area. Eggs and baby animals are easy targets for predators and often fall prey to other larger animals.
Seagulls are not the only animals to do this. For instance, herons and everyone’s beloved swans also kill other animals’ chicks, either for food or due to resource competition.
In summary, it’s not pretty but it’s just the way things are in nature.