Macbeth is a famous warrior and used to killing people in battle. Why is he so frightened and distracted in Act II scene 2 of Macbeth?
Let us remember that Act II scene 2 is the scene in which Macbeth emerges from Duncan's bedchamber having killed him in cold blood. You are right to spot the contrast between Macbeth as we hear about him in Act I, where he is described as being "Belladonna's bridegroom" and also when he kills people very savagely in battle, but we must remember that there is a massive difference between killing people in the heat of battle and then killing somebody whilst they are asleep. Let us also remember that the crime of regicide, or the killing of a King, was thought to be a terrible crime to commit, as the king of a nation was believed to have been appointed by God himself. Killing a king therefore meant that you were defying God's will, and going against God is never a good idea! I think we can also add a third reason, which is that in this scene Macbeth commits a crime which could have massive consequences for his own life and personal safety. When he killed enemy soldiers, that was sanctioned murder, as it was in battle and they were fighting against his king's rule. Suddenly, he is killing somebody without saction, and if he is discovered, his life will be ended very quickly.
Hence we can understand why the aggressive and belicose Macbeth is presented as being such a nervous individual, concerned about his actions and why he couldn't say "Amen" in response to the prayers of the grooms:
But wherefore could not I pronounce "Amen,?
I had most need of blessing, and "Amen"
Stuck in my throat.
This is also why Macbeth is perhaps unable to face the murder scene again, and sends his wife in to smear blood on the grooms.