Nolan only did what he did in the end of Season 1 because basically he’s trying to convince himself he still believes and is loyal to Viltrum. He just needs Mark to validate him, which he doesn’t get, and that’s why he breaks like he did. He doesn’t actually see Debbie as a pet, nor does he actually see human life as pointless. That’s the whole point of it. He’s trying to sell himself on all of that.
Pretty sure they can make humans live longer through giving them better health conditions and expand their life expectancy, but they very much cannot stop a person from aging. They wouldn’t even have an incentive to do any of that, they see anything that lives less than them as weak and insignificant anyway