I was wondering, and came up with this theory, Do you think MX used ven to get terra to join him and then when terra went to MX, he corrupted him and changed him. I think MX was talking about terra when he said, already that man is a monster infused with darkness, defeat the darkness and correct my mistake. I think of this as sort of like when a mad scientist thinks he has created the (ultimate project or weapon,creature,etc.) then he slowly begins to realize he has made a grave mistake and created a monster, and now wants that 'mistake' to be stopped. do you think MX thought that terra would be a great apprentice for him, but realized terra was being over taken by the darkness. Maybe that's why he said one who submit's to darkness is not qualified to wield the keyblade. (On a side-note): Another short theory, Do you think terra knew he wasn't strong enough to defeat MX and maybe he chose riku to wield the keyblade because he thought that maybe in the future, riku might be able to beat MX,( but, then riku gave in to darkness and sora got the keyblade.) just a thought.