What evidence from the text supports the idea that Lady Macbeth questions
her husband's ability to kill the king?

B.  Art not without ambition, but without / The illness should attend it.

Explanation:

In lay terms, she's saying "You want to, but you are not cruel enough."

In the late 16th and early 17th century, the word illness referred to wickedness, depravity, or evil conduct.  It actually wasn't until 1692 that it was first used to refer to a sickness of the body.  

If your institution provides access to the Oxford English Dictionary, I highly recommend using it to gloss your text as you read Shakespeare—it's tremendously helpful.

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