Sora just kills every heartless he sees without hesitation or exception, and he's killed thousands of them. If you think about it, isn't the technically considered genocide?
Nobodies can plan, and think. They do not possess emotions, however. Dusks and Shadows are pests. Sora, keep on killin/slayin/doing somethin to get red of em!
Heartless are mindless. Sora saves them by killing off the darkness in them. See, when he destroys a heartless, he doesn't really "kill" it, but releases its heart.
"Pitiful Heartless, mindlessly collecting hearts. The rage of the Keyblade releases those hearts. They gather in darkness, masterless and free...until they weave together to make Kingdom Hearts. And when that time comes, we can truly, finally exist."