Is excessive ambition the only source of Macbeth's "evil"?According to the critic L.C. Knights, "Macbeth defines a particular kind of evil the evil...

Well, I'd start by asking - what is Macbeth's "evil"? Knights makes a couple of significant assumptions: first of all, that evil as a concept exists not only in our world, but in the world of Shakespeare's play. Secondly, he argues that a "lust for power" automatically leads to evil. I'd challenge, and disagree with, both of those assumptions.

Firstly - is Macbeth really evil? He seems to be a popular soldier, at the start of the play, he's pretty much single-handedly won a huge battle for his country against the Norweigans:

The King hath happily received, Macbeth,
The news of thy success; and when he reads
Thy personal venture in the rebels’ fight,
His wonders and his praises do contend
Which should be thine or his.

He has a close friend in Banquo, and there's no evidence, anywhere in the play, he's ever done anything wrong before. Duncan seems to think he's fantastic (and that's no evidence, thus far in the play, that he's a bad judge of character).

At the start, Macbeth isn't evil. And if he were evil, surely there would be no tragedy? Tragedy happens when you connect with the material, when it takes you on an emotional journey with it. And there's nothing sympathetic or interesting about a monster.

He is, of course, ambitious:

Two truths are told,
As happy prologues to the swelling act
Of the imperial theme!—

He is also persuaded by his wife that he should kill the king. He is also the subject of a series of prophecies by the witches on the heath, that tell him he is going to be the king. And the fact that his desires for the crown are "black and deep" suggest that he has longed for the crown before even the prophecies are spoken. But all of that strikes me as irrelevant to your question, really.

Macbeth isn't "evil" at the start. But he becomes evil. He becomes a "dead butcher", in Malcolm's words, killing everyone and losing all empathy. And how does it happen? He makes a mistake. He is persuaded - partly by his own ambition, partly by his wife, partly by the witches' prophecies - to kill Duncan, his friend and the country's king.

After that he himself is on the throne, and there is never a secure moment. He worries, he panics, he can't sleep. He is forced to kill again, and again to desperately cling at security. And he becomes something evil: his mind is "full of scorpions".

So his ambition is one factor in what makes him make a terrible mistake. But that doesn't itself make him evil: evil is at the bottom of the slipperly slope which his ambition - along with the other factors above - pushes him over the top of.

Hope it helps!