It is a very different story when one kills a predator to make a new safe living space for themselves and their family. The person in question had a safe and comfortable place to live and went out of their way to kill a sentient being and then left the area thus secured.
One is about necessity and the other is a blood sport.
Also where does it say that humanity have to spread? I’m sympathetic with people whose current location become undesirable and they have to move to a new place. But “spreading” is not the goal.
Society uses, builds, and works to maintain delusions like this because it helps keep us all from a mad max future. It’s one of those ‘useful to believe’ things most of the time.
But don’t buy it so much you let someone use it against you as a weapon, instead of as a mutual agreement.