God to bless him too, but cannot bring out the simple word "Amen." It stuck in his throat at the time, and all the words "'Amen' stuck in my throat" should be uttered by the actor in a hoarse rasping voice.

Macbeth does not understand why he could not pronounce that word, and his wife really doesn't care because she has more practical matters to think about; but the audience understands that God will not allow him to pronounce it because he has committed an unpardonable sin. Later, in Act 3, sc. 1, when he is planning to have Banquo murdered, he confirms that he has sold his soul to the devil for nothing:

For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewelGiven to the common enemy of man
To make them kings, the seeds of Banquo kings.

It was really very inappropriate and presumptuous for Macbeth to be saying "Amen" to one of the grooms' "God bless us" while he is standing there with his "hangman's hands" covered with Duncan's blood. It is like asking for forgiveness practically at the same moment he is committing the crime and when he should know he has committed an unforgivable sin. He is foolishly and vainly hoping that somehow everything will come out all right.

"Amen" is a well-known word with religious significance. It has been around for thousands of years, and is used to show agreement and confirmation of something or some point which is why it appears at the end of so many prayers. Congregations confirm what a priest or preacher has just said or what they have said in unison, verifying the truth of what has been said, its grounds and their agreement. In a modern context, outside of its religious context, those in agreement with each other may say something like "Amen to that" when they want to express their consensus. In Macbeth, Macbeth is a respected and loyal subject of Duncan. At first, he is even shocked at his own callousness and intent. Having decided not to proceed with his plan to murder Duncan, he allows himself to be persuaded by Lady Macbeth that he should go ahead with it and Lady Macbeth convinces him that it will reveal that he is "so much more the man" (I.vii.51). 

In Act II, scene ii, Macbeth kills Duncan and relates to Lady Macbeth the disturbing comments of Duncan's aides, whom they intend to blame for Duncan's death. Macbeth expects them to remain asleep but when they see his bloody "hangman's hands" (28) and one cries, "God Bless us" (27), Macbeth is so conflicted because he knows that God will not spare them as Macbeth will kill them to save himself. Therefore, for Macbeth to say "Amen" at that point would be to lie in the face of God because his wish would be for God to save only him. He does not want them spared for obvious reasons, as they will be able to reveal his guilt. So he cannot agree with them by uttering the common "Amen."

This also reveals the extent of his superstition. If he agrees with them, perhaps they will be spared by some miraculous intervention, and that will certainly be catastrophic for him. The fact that "Amen...stuck in my throat" (32), and that he later reflects on this, foreshadows the disastrous chain of events that will follow. He admits that "I had the most need of blessing," and now, having been unable to agree with their prayer, he may have doomed himself.