From my view, it's a matter of perspective if a person has an iron will to resist further temptations should one decide to cross the line, trying not to descend into darkness. Hal Jordan ending also highlighted that when he decide to go covert as Yellow Lantern, even though he just got back his Green Lantern power. As sometimes choices made would have its consequence - cause & effect karma. I think you would hate Buddhism philosophy in the "no killing" thinking from the Mahayama teaching.
Batman do have a point, though. Once Joker is killed, someone will take his place & will be worse than Joker himself. Arkham Knight kind of highlighted that, that Scarecrow would turn out to be a much more terrifying villain in the large scale scheme.
But either way, in Injustice, Superman fallen from grace was ultimately Joker's victory in making Superman a villain worse than himself.