Mostly because Humans are the only ones that love killing each other.
Dont see prides of lions killing the pride next door just for shits and giggles
Sure they do, if you count sex. Male lions will kill rival males in other prides so they can take over mating rights. Both male lions & female lions will kill the cubs of rival prides.
I think you missed GP's point. The behavior you describe is the result of selective reproductive pressure over millions of generations of lions. It is hardwired into the lion's genome, and is quite rational behavior for any organism looking to optimize its reproductive probabilities. . But sometimes humans kill even when it doesn't optimize their reproductive probabilities. As far as we know, humans are the only organisms that kill for sport.