Before Macbeth kills Duncan, the king, he is nervous and already feels guilty. You can best see this in the part (in Act II, Scene 1) where he has the vision of the bloody dagger. This clearly shows that he is uncertain about what he is about to do.
After he actually kills Duncan (Act II, Scene 2), he feels even more guilty. He believes that he has heard people accusing him of murder. He also finds that he cannot speak the word "amen" when he tries to pray. Both of these show the depth of his guilt.
In the play 'Macbeth' by William Shakespeare, Lady Macbeth and the witches have wound her husband up into a frenzy of ambition and desire for power. Before killing Duncan, he is more motivated by these desires than by a desire to kill or murder for it's own sake. He has nothing against Duncan other than that he stands in his way of getting those things. By the time he is ready to do the dirty deed and actually kill Duncan, he has retreated into himself and become obsessed with one idea alone. We call this becoming fixated. His fixedness pushes everything else away. He pushes every thing and every one else out - including his wife Lady macbeth. He has become unreachable by reason or moral thought and his relationship with his wife has changed - they are no longer a couple - and he takes over the momentum alone.