It's not his ambition that ruins Macbeth, it's his wife's ambition. She's the one hungry for power and glory, not Macbeth. She infects him with her ambition. And then there's this:
The witches greet Macbeth with two truths and one prophecy. They tell him that he is the Thane of Glamis, which everyone knows already. That's the set up, part one. Then they hail him as the Thane of Cawdor. He's already been named that, but the news hasn't officially reached him yet. That's the second part of the set up. Then they hail him as "King hereafter." That's the prophesy.
After the witches vanish, Macbeth and Banquo meet up with Ross and Angus. They bring news that he has been named Thane of Cawdor. The trap is sprung. Banquo says, "What, can the devil speak true?" And now Macbeth believes that what the witches said is a promise. And that's the second ingredient in Macbeth's downfall: he's gullible and is easily manipulated by others' words and by his own imagination.
Now, can you put together your thesis statement with that information?
I think that the most likely thing for you to talk about on this particular subject would be the witches' prediction or predictions. Their predictions (especially the first one, given on the battlefield) have a great deal to do with amibition (specifically, with Macbeth's ambition).
My thesis statement on this topic would be something like this:
In this play, prophecies and predictions greatly affect Macbeth's ambitions. Before Macbeth and Banquo meet the witches, Macbeth is not particularly ambitious. After they meet the witches and hear their prophecies, he becomes very ambitious. The ambition eventually destroys him.
With the ideas of the above posts in mind, consider using the witches' paradox, "Fair is foul, and foul is fair," as part of your thesis. For, throughout the entire play, the characters are under the shadow and menace of these opening lines. Playing upon the English proverb, "Fair without but foul within," Shakespeare's Macbeth has the reechoing of this proverb in the words of the witches as descriptive of Macbeth's actions and his deceptions both.
For instance, Macbeth himself nearly says the words of the witches in the opening scene: "So fair and foul a day I have not seen"; thus, he establishes his connection with the weird sisters who echo his words paradoxically, establishing the discrepancy between appearance and reality. It is this discrepancy between what is fair and what is foul that disarms Macbeth as he makes "evil his good" in his ambition, and destroys Lady Macbeth, who becomes "foul within."