why do you think keller killed himself?

I think Keller can't handle both his sons accusing him of the crimes he committed. He even feels the accusations of one son from beyond the grave and his other son is ready to send him to prison. In act 3 Chris talks about putting his father in jail,

" I could jail him! I could jail him, if I were human any more. But I'm like everybody else now. I'm practical now. You made me practical."

I also think his personal guilt has also been building for so many years that he just could not live with himself anymore.