It sounds to me like you need to provide a thesis statement that answers the question of whether or not internal or external factors are most to blame for Macbeth's downfall.  Don't get hung up on trying to decide which is the right answer.  There isn't a correct answer to that question.  It's completely reader opinion.  That's what a thesis statement is anyway.  It's a statement of opinion that your essay/paper is going to try to prove.  

You could write a thesis statement that states your opinion followed by a couple of reasons.  For example, you could write the following: "Macbeth's downfall is caused by the external influences of his wife and the witches."  The thesis states that you think external factors are to blame, and then it specifies which factors are most to blame.  The paper will then explain to your readers how Lady Macbeth and the witches are the root cause of Macbeth's downfall.  

Personally, I like thesis statements that force writers to write about both sides of the argument.  It will typically start with a dependent clause.  For example: "Although Macbeth's downfall is influenced by external factors, his own internal ambitions and actions are more to blame."  This kind of thesis statement will force you to write about the external and internal factors.  It forces you to recognize both sides of the argument and steer your reader to believing one side more than the other.  It's a very strong thesis statement format.    

I agree with your assessment.  Macbeth is largely responsible for his decisions and actions but is externally influenced by Lady Macbeth and the witches.  Here is how to organize this paper: You could set up the thesis in a way that would focus one body paragraph on each of your main character ideas:  1) Macbeth's personal role, 2) Lady Macbeth's influence, and 3) the witches' role in Macbeth's downfall.   Organize the body paragraphs in a way that would address Macbeth's role in the final body paragraph, as that paragraph seems the most significant and powerful. 

Possible thesis ideas could revolve around how Shakespeare challenges the idea of free will or culpability or self-responsibility through the play Macbeth, particularly in Macbeth's struggle with his own ambitions and the external influence of his wife and the witches.