There's also this theory that the element they have, "killed" them in the first place(whoa, did anybody get any of that?). Like with Axel, some say that he losed his heart during an accident by fire.
But it can also be the same thing as the keyblade. The weapon chose its wielder.
(I'm sorry about every possible error in my english)
...*thinks*...wait a sec... Riku and mister. Log are a couple?... xD *insert crazy laughing here*. Of course, a new pairing has been made! Rikuxlog! xD
With the little stuff, it's like "kill off everything that moves". With bosses I usually look what kind of moves they have and where and when I can hurt them. Then they are usually dead meat (öh, Sephiroth? Well, he can wait...)
Saw it on Aku Ankka (Donald Duck magazine in Finland), and I was like OMG, spiky haired dude with a huge key!? I got the second part of the series first, year ago, but heard about the game MUCH before it.