Why did Ichigo use his final move?

Question

At the end of the Aarancar arc, Ichigo had Aizen so completely outclassed that Aizen couldn't even grasp the difference in their powers.

Why would Ichigo use his Final Getsuga Tenshou when he knew what it would cost him?

Answer 1

Because he wanted to finish Aizen for good.

Aizen still had the Hogyoku, and even after Ichigo's attack with Final Getsuga, he was able to regenerate (chapter 421), while Ichigo lost all of his powers due to that very attack.

Even Urahara states (later in the same chapter), that it is "nearly impossible" to kill Aizen while

he is merged with Hogyoku. WHen Ichigo loses his powers, you can see Aizen getting up, ready to finish Ichigo off. At that moment, neither of them knows about what Urahara has done, and Ichigo is surprised and shocked by the fact that his opponent is still alive after his (Ichigo's) most powerful technique. After that, Hogyoku finally rejects Aizen, and Urahara's seal activates, basically saving powerless Ichigo.

So I'd say Ichigo had to use it because he saw it as the only way of truly defeating Aizen.