The only Disney villain to actually become a heartless is Scar. The keyblade is the most effective weapon against heartless as well as the only thing capable of releasing their captive hearts. Beings that are not heartless are not specifically weak to one, but that doesn't mean that hitting them with a blunt object wont do some damage.
In the manga adaption several Organization deaths were altered. Larxene dies because Sora, Donald and Goofy get her wet so when she uses her Lightning attacks she fries herself and that kills her. Xaldin dies after the fight with Sora when he escapes and a Vexen replica kills him. (Vexen makes several Replicas of himself during the CoM manga adaption, and for some reason one decides to kill Xaldin)
It is important to note that the only person Sora has killed on hs own is Luxord, every other major battle he has particpated in he has had some form of support.
(Also Kefka did not absorb the statues, only the magic that they were exchanging with each other as well as that of several espers. You fight the three statues before taking down Kefka himself)
