Sure Genocide had just had her spine torn apart and her limbs mutilated, but that by no means renders her defenceless or stops her being any sort of threat. She's some flavour of undead godling with an utterly toxic memetic effect that can sunder souls and corrupt the very stuff of the heavens itself, made out of her own corpse.
It's not vengence, it's an act of social conscience and responsibility, and yes even supreme mercy, to kill it and hurl the remains into the sun.