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.