The "winner" of the Keyblade War would eventually open Kingdom Hearts and create a new world where Light & Darkness coexist in balance.
Point is, MX is kind of the good guy when you see it that way. He wants balance. To create balance, he must start the world over. Makes sense, right?
To that end MX is a bit of an antihero.
^^^In truth I believe that was the original intention of MX but then I think he lied all about having balance inorder to get power.
I think it was the other way around. Once he started gaining power, he didn't care about his original intentions and just wanted power. Darkness had corrupted him so much that he went on to believe it was the heart's true essence, even though he said he believed in a balance of light and dark early in his reports.