At the beginning of the play, we see Macbeth as a valiant and loyal soldier who deserves all the praise and admiration heaped on him by everyone. However, since it becomes evident that he starts harboring ambitions of becoming the King of Scotland, his life becomes very turbulent and agonizing. Once he kills Duncan, he understands that his life will never be the same again. He will never be able to regain peace and stability:
Methought I heard a voice cry 'Sleep no more!
Macbeth does murder sleep', the innocent sleep,
Sleep that knits up the ravell'd sleeve of care,
The death of each day's life, sore labour's bat...
He begins hearing voices and experiencing dreadful visions which torment him profoundly. All of this is the result of his guilt-ridden conscience. For instance, the scene which depicts Macbeth's fatal vision of Banquo is especially striking as it lets us know how tormented Macbeth's mind has become.
Once he becomes the king, Macbeth's fears and frustrations do not recede. He becomes obsessed with his ambition to retain the power he has as the King of Scotland and shows how insecure he is. As a consequence, he orders the executions of many innocent people. By the end of the play, he has become a cold-blooded murderer, whose guilty conscience is deadened. He finds life meaningless, saying it is "a tale told by an idiot, full of sound and fury, signifying nothing."