Isaac Arthur, and the Orion’s Arm Project, are what inspired me to create the Office of Termination setting, and the same reason could be used for justifying settings where death is casually carried out.

If you live in a society where interstellar travel, even without FTL, is possible, along with extreme life extension, mind augmentations, advanced automation, artificial wombs, and so on, you could have an individual or small group creating a colony in an unclaimed star system. They could condition, or genetically engineer, the population they create to become comfortable with casual death. The casual deaths could be random termination, purges, violent games, or whatever else the ruling entity desires. Hell, the being could partake in some of the killings.

The best thing is that the entity doesn’t need to at all justify itself, but they can if they wants. The population reaching zero isn’t an issue, since they could always grow more people in artificial wombs. Even if the people are not genetically modified to accept their deaths easily, they are at the mercy of a being that has the resources of an entire star system to crush any rebellion without breaking a sweat; hell, the being could block out the sun, or direct enough sunlight to fry everyone on the planet. If the population is living in rotating habitats built by the entity, then a rebellion is even more fruitless.