I think an excellent title for your essay would be: The Illness of Ambition: Characters Reactions to Ambition in Macbeth.
This title is taken from a quote from the play. It is often good to name an essay with a play on a quote from the story.
Ambition is a good subject for an essay on Macbeth. Without knowing what your specific direction is, I will comment on some titles that could work for the theme.
Lady Macbeth is the driving force of the play. She is the impetus of the action. She realizes that Macbeth could be spurred on to do what needs to be done.
Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene V, p. 19)
So, he can do it. She wants it. He just needs a little push. She pushes him. This quote also reminds us that Lady Macbeth is the one doing the pushing, but Macbeth is clearly pushable.
This is a good title for an essay on ambition because it uses Lady Macbeth’s idea of how to develop ambition into action. Of course it takes illness to take the worse steps, such as committing murder to get what you want.
When coming up with a title, it is good to connect it directly to your thesis. If you can relate it to a quote that is an added bonus. I hope this gives you a good idea!