Kingdom Hearts keyblade of peoples hearts

why is it that when 7 hearts of pure light come together...they make a dark keyblade? that makes no sense

EDIT: 6 hearts, kairi wasn't included

 

The keyblade was incomplete withou Kairi's heart. It only contained the hearts of the first 6 princesses' hearts. http://images2.wikia.nocookie.net/__cb20101011143649/kingdomhearts/images/f/f0/Keyblade_of_People%27s_Hearts_KH.png As you can clearly see, the teeth of the keyblade is a incomplete heart.

 

Riku, who was possessed by Ansem SoD was like the pinnacle of all darkness.
It probably wasn't that hard for him to manipulate the Darkness, which lets face it, he had a lot under his control, and then manipulate the Princesses Hearts (which yeah, were pure light) into a Keyblade somehow. I don't know the process... I mean, who does? lol
but yeah, that's how I view it anyway.


EDIT: Also as Aerith said... the keyblade was incomplete without Kairi's heart which as well all know, at the time resided inside Sora.

 

^this

And when Sora got turned into a heartless, Kairi was the one who brought him back. But uhm, yea the keyblade was pretty much incomplete without Kairis heart.

 

Okay you're asking why does 6 Hearts of the purest light create such a dark weapon?

Simple, Riku only obtained it when Ansem took root within him as that is the Keyblade was introduced, my guess is, he use the purest hearts and twiste dtheir power to become dark and create it as it is a very magical weapon, we don't know how the Keyblade was made but I suppose 6 Hearts of the purest Princesses in the universe nearly makes it rival that of the Keyblade's power, of course, it was uncomplete since Kairi's heart was within Sora's, so when Sora used it to free her heart, his innocence or light helped free the captive hearts at teh cost of himself had it not been for Kairi's power as a Princess of Heart and her love her Sora that he returned,

I suppose you could call Sora when he came back from a Heartless was his body and soul while Roxas had Sora's Heart since Xehanort was two different people when he experimented and became a Heartless and Nobody but when Sora beat them, Xehanort is said to return in Coded's Secret Ending.

I'm getting off-topic here but if Kairi's Heart did join with the others, a different form of teh Keyblade could of been made or a false one.

The X-Blade also similarly works around hearts however, one of pure light and one of darkness so it's just two but Ventus and Vanitas were two halves, each wielding their own true Keyblades.

 

This is a spoiler of my thoughts from BBS that might shed light on this.

Spoiler

So back then there was Master Xehanort. He took over Terra's heart but got trapped in it with him which led the both to be stuck there in Xehanort who was Ansem the Wise's apprentice. He turned into a Heartless which is Ansem SoD. Master Xehanorts heart was with Ansem SoD which is why he looks like that and has that shadow behind him. Since Master Xehanort had a keyblade and was a master of darkness and was trying to find pure hearts filled with light he should know how to manipulate them since he was able to manipulate Eraqus's light.

I wouldn't read this if you don't want BBS spoiled. Sorry.