What would be a good thesis statement for an essay on the topic of Macbeth being evil in the play?

I don't know if your thesis statement has to begin with Macbeth being evil in the play. If not, I would change it to "Macbeth's ambition drives him to evil acts in the play with unfortunate consequences for himself, others and the kingdom." But if you have to word it the way you describe, you could say that "Macbeth, being evil in the play, gives in to his overwhelming ambition, causing death and destruction throughout the course of the drama." Then, of course, you would look for examples of ways Macbeth's ambition leads him into evil acts, such as murdering Duncan, Banquo, etc, and the effect this has on him and others. You might also want to contrast what Malcolm has to say in Act IV, scene iii about the characteristics of a good king with Macbeth's lack of ability to control his impulses—in other words, Macbeth's ambitious streak as a contrast to the humility and honesty of the good ruler.