Birth by Sleep Desperation?

When you fight Terranort, at the end of the battle, MX tries to move but Terra keeps him from moving the body. At this point, MX, in an attempt of desperation, unleashes the power of his keyblade upon his heart to get rid of Terra. However, this action causes both of them to fall into the darkness, and amnesia takes place.

My question is: If MX is such a meticulous fellow, who already states that he made many paths to ensure victory on his behalf, how come he is so naive to think that "releasing" Terra's heart will not affect him at all?