It is interesting to note that Macbeth feels far less guilty about killing Banquo and Macduff's family than he did about killing King Duncan. This is partly because he doesn't have to do the actually killing himself but can delegate the work to others. But something about the experience of killing the King with his own hands, and then having to go through the nightmare of being present when the body is discovered and everybody in the castle is awakened, has evidently hardened him. He is a different man for the remainder of the play--a man without a conscience and without a soul. He has already committed the worst crime imaginable, so his further crimes are comparatively easy. He says of himself when thinking of having Banquo murdered: "...and mine eternal jewel / Given to the common enemy of man." In other words, he has sold his soul to the devil in order to fulfill his ambition to become king and to make his beloved wife queen.
On the surface, Macbeth appears to be ruthless and cold about the killing of Banquo and Macduff's family, but his bloody actions are actually driving him insane. When Macbeth hears that Fleance has escaped, Macbeth says:
"There the grown serpent lies. The worm that’s fled/ Hath nature that in time will venom breed; /No teeth for th' present" (III.4.30-32).
Macbeth likens Banquo to a grown serpent and Fleance to a worm that will eventually grow into a serpent, and Macbeth feels relieved that Banquo has been killed. However, he begins to see ghosts about him at the banquet of noblemen, and it is clear that he has been pushed to insanity by his ruthless actions.
After Macbeth kills Macduff's family, Angus says about Macbeth:
"Now does he feel /His secret murders sticking on his hands./Now minutely revolts upbraid his faith-breach./Those he commands move only in command, Nothing in love. Now does he feel his title/Hang loose about him, like a giant’s robe/Upon a dwarfish thief" (IV.2.17-23).
In other words, Macbeth has begun to feel horribly guilty about his actions, and, as a result, he criticizes his soldiers. He has become unfit to be king, and Angus compares Macbeth's rule to a giant's robe that Macbeth is too small to fill. Macbeth has begun to suffer psychologically as a result of the murders he ordered, and he has begun to lose his sanity.