I think the sad part was Mike feeling like he needed to hit himself in the head with a rock for the sake of the others.

Oh whoa I thought you were going to be hate thanks for not being hate!

I agree. Yes, that is the sad part

And not only that, but that he is so incredibly desperate to free himself of TMO that he would go to extreme measures. 

He had learned to cope with MPD when the personalities were familiar and friendly, and just as he learned to cope with that everything changed. He suddenly lost access to parts of him that had become almost like friends to him, and he suddenly lost that control he had only just gained over his life. He must feel so fundamentally alone. And confused. And scared. 

He lost control of himself and desperately tried to regain that control by self injury

And that’s scary. And that’s real.