Oh, a fun question!
Consider the circumstances surrounding Duncan's death, and what the death means for the country. That should generate all sorts of questions.
Personally, I'd start with the fairly polite and information gathering, then sneak in. That means asking things like "Mrs. Macbeth, we understand that the king of Scotland was found dead here this morning?"
(She says something like "That's correct.")
"It must be terribly traumatic for you to have something like this happen nearby. What was it like?"
"Do you feel safe yourselves? Can you sleep at night?"
(Then I'd start sneaking in.)
"I understand that your husband discovered the alleged killers of the king and killed them. Was he himself injured in the process?"
"You keep rubbing your hands; are you okay Mrs. Macbeth?"
"What do you think this will mean for the country? Really? That's a surprisingly positive idea. Why do you think your husband would become king?"
"And do you think Scotland will accept someone so closely involved in regicide?"
"Where is your husband now? Why do you keep rubbing your hands…?"
And so on.