I'm just a we bit spot on!
Out of all the tragedies Shakespeare has written, MacBeth turns out to be the closest in relationship to himself. Even though its just about the simple rise and fall of a man. What makes MacBeth different from his other tragedies is there are no humorous moments, or self reflection that take place in his the other 2 tragedies Hamlet and Romeo & Juliet. Actually, there maybe one dark humorous moment and that occurs in the scene of Porter, a bit argument-able in MacBeth, but that's about it.
Before i get ahead of myself, what is a tragedy? A tragedy is where the main character(s) have a flaw that ultimately brings them to their own down fall. A tragedy will also reinforce values from the time period in which the book was written from. With that insight then the break down of MacBeth comes easily.
We see that in MacBeth with the advancement in isolation comes a decline in moral and mental health. However, it is resulted from MacBeth's own decisions. That bring him to his own demise. It is that unwise decision making though that draws in the audience.
Why does MacBeth kill duncan?
You could say Ambition, but thats to simple. MacBeth kills Ducan because of the witches prophecy. The witches prophecy however only told Hamlet what he already long desired. We assume Hamlet already had a desire to become king, because he does not question the witches like Banquo did. He accepts it and allows it to influence his decisions to come, like killing King Ducan. Which in case you didn't know violates everything his community believes in.
Ambition and Immorality Create the tension in MacBeth. So here comes the MacBeth and Hamlet comparison. In Macbeth, like in Hamlet MacBeth struggles to act upon his thinking.
As for Lady MacBeth she has no conscious to rule over her. She's just a tool of destruction or in other words a self righteous hoe. Ultimately she ends up killing her self due to the lack of inner will and an over whelmed feeling of guilt for killing king Duncan, which reminded her of her dad.
Macbeth as a king..? Once Macbeth became king it was the classic slipper slope, downfall, demise, what ever you want to call it.