That's because he was the only one who hold out long enough to allow his innocence to melt. Also the only one effective enough t think of it.

I have a lot for respect for Hoshino because of the way she portrays the sadness and futility of war ( Not like where there was an 'epic war' that everyone survived) I thought she had a lot of guts to stick to that and kill of characters I think I'll lose some respect fro her if she brings any of them back.