The Five Stages of Grief
It was his fault that Tadashi was gone. Hiro knew this. Every moment, waking or sleeping, was punctuated with his own inner voice sneering, "If only you'd tried harder to stop him," or "If only you'd not made those stupid nano-bots."
Aunt Cass said he wasn't responsible, that no one was. Tadashi was a hero, and "we should be proud."
"You should have gone in after him," said the voice.
Then we'd both be dead, Hiro countered.
His innermost thoughts scared him with their answer: "Would that have been so bad?"
Don't let this be real.
It's all my fault.