After visiting Duncan's room and hearing one attendant exclaim, "God bless you," Macbeth is unable to say, "Amen." Why does this concern him?

Macbeth knows he is committing the ultimate sin by stabbing Duncan a dozen or so times. Macbeth tries to say Amen along with the prayers in the next chamber but can't. Macbeth knows he is going to Hell and can't seek any absolution from God for what he is about to do.