"But wherefore could not I pronounce"- What was the thing that Macbeth could not pronounce and why?

In Act2 Sc 2 of Macbeth, a great tragedy by William Shakespeare, Macbeth could not pronounce the word, 'Amen' just after the murder of Duncan.

             'Amen' is a holy word. Macbeth cannot accentuate it because his mind is sorely convulsed. He is in dire need of divine blessing but he become chocked. This is an astonishing revealation of his mind. He is remorseful and repentent for the heinous crime of regicide. He has understood his unpardonable blunder of committing such a gross injustice to the monarch. He is extremely repentent for his misdeed and he earnestly wants to expiate for it. But his soul is so burdened with sin and evil that he is unable to speak 'Amen'.