Macbeth's perilous ambition to become the King will drive him to commit many evil acts which will ultimately lead to his downfall. Once he has the idea of becoming the King solidified in his mind, he does anything he can to make it possible:
Stars, hide your fires;
Let not light see my black and deep desires...
Indeed, his "black and deep desires" lead him to commit hideous crimes directly or indirectly. When he kills King Duncan and has Banquo killed, we see that he is more than resolute to make sure that he becomes the King and remains the one for good. The only difference is that, at the beginning of the play, he is victimized by his guilty conscience, while towards the end, he becomes a ruthless murderer, ready to kill anyone who stands in his way:
I have almost forgot the taste of fears;
The time has been, my senses would have cool'd
To hear a night-shriek...Direness, familiar to my slaughterous thoughtsCannot once start me.