Macbeth is about struggle between good and evil in an individual. The power of evil is so destructive, and if evil is allowed to override one's conscience, one has to grapple with devastating consequences, as seen in the play.
There are so many aspects of evil which you can focus on, so your thesis depends on what you wish to analyze in particular. I would suggest examining the birth of evil in Macbeth and its impact on him. So my thesis would look like this -- Macbeth implies that evil is not only present in the supernatural but also in the depths of the human heart. Therefore, if it is allowed to surface, it can destroy an individual and the world around him.
You could focus on the birth of evil in Macbeth and its impact on him. Once Macbeth succumbs to the devastating power of evil, he is unable to break away from it. His soul becomes poisoned. Once he accepts evil, he is conquered by it.
The play teaches us that evil is not compatible with humanity and if one embraces it, one is bound to be destroyed. This is what happens with Macbeth and Lady Macbeth at the end of the play. Lady Macbeth is plunged into madness and death by her guilty conscience. As far as Macbeth is concerned, he realizes that his life is reduced to "a tale told by an idiot...signifying nothing." His actions, controlled by evil, become meaningless after all.