IS natural selection cruel?
Life is red in tooth and claw, with predator species and prey species. In successful ecosystems predator and prey keep each other alert and ready.
Every dog, cat, squirrel, and chipmunk will die. Some will manage to die of old age, which isn’t all that kind to endure. Many will die at some point from a predator’s bite. That is very cruel, but usually pretty short.
Define cruelty first, then realize that evolution and ecosystems are build around death. Life without death results in starvation when all available food has been consumed by all the new members of the deathless species. Ewwwwwps!