Who kills the organization in Kingdom Hearts

Sora kills off a large proportion of them in both Kingdom Hearts 2 and Chain of Memories. Here they are in roughly the right order:
Xion is defeated by Roxas, who is unaware of his actions
Vexen is incinerated by Axel
Larxene is defeated by Sora and fades into darkness
Marluxia is defeated by Sora and has an epic death sequence
Lexeaus is slashed through his stomach by Riku Replica, who is possessed by Ansem
Zexion is strangled and drained of his life energy by Riku Replica, who is being manipulated by Axel
Demyx is defeated by Sora
Xaldin is defeated by Sora
Xigbar is defeated by Sora
Luxord is defeated by Sora
Saix is defeated by Sora
Roxas merges with Sora, so does not technically die
Xemnas is defeated by both Sora and Riku