How do I make a comparison between Macbeth and The Godfather?

Macbeth and The Godfather are both about power. In each case, someone rose to power quickly and not necessarily expectedly. The road to power became increasingly bloody, and it had consequences for the protagonist's loved ones.

In Macbeth, Macbeth hears a prophecy that he will become king. Until this point, he was just a loyal soldier.  Since he was not a son of King Duncan, being king was probably not on his radar. Once the possibility was there, however, Macbeth became obsessed with it.

MACBETH

[Aside] The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires (Act 1, Scene 4).

Macbeth was advised to try to become king by Lady Macbeth, who was bloodthirsty and ambitious enough to talk him into killing Duncan. Her plan was successful, and Macbeth became king. They had to kill Duncan and the guards, which affected both of them. Over the course of the play, they slowly succumb to guilt. 

Michael Corleone’s rise to power was similar in that it was unexpected. When his father was attacked, he seemed to be the most qualified to lead. A gang war had erupted, and Michael wanted to kill a corrupt policeman to save his father. He also had to retaliate against the gangsters who targeted his father and cement himself as the head of his family. 

Both Macbeth and Michael were faced with escalating violence. Macbeth had to kill first Banquo and then Macduff’s family to maintain his position as king. Michael had to kill the corrupt policeman and rival gang members. Both men went from fairly normal lives to thinking murder was not a big deal. Both had to kill people close to them. Macbeth feared Banquo would betray him, and Michael found out that Tessio and his sister’s husband both betrayed him, meaning they had to die. 

There are differences in the stories, of course. Michael’s girlfriend had no idea what he was. Even after she married him, she didn’t realize he had become a Mafia kingpin. Lady Macbeth was completely complicit in Macbeth’s rise to power, but Kay was an innocent bystander in Michael’s. 

Macbeth lost his power by the end of the play, but Michael cemented his. Malcolm came after Macbeth; Barzini came after Michael. Michael had good advisors, intelligence, and a strong survival instinct. By the end, he lost his brother and his father, but he was on top. Macbeth lost his will to live as he became more and more unstable. When facing Macduff, he gave up when he found out that the prophecy about bewaring Macduff was relevant since Macduff was not born of woman.