Okay, I was misremembering.
And apparently Macbeth does end up killing Duncan even tho at one point the plan was for Lady Macbeth to kill him.
But it's a cool quote, nonetheless.
Nice couple, the Macbeths...not.
I had to find an online synopsis, because I haven't read it since High School, but Lady Macbeth is the driving force behind the deed. Mac has some doubts about following through, but she calls him a coward and tells him to "screw his courage to the sticking point".
"Behind every great man" and all that, I guess.
She mostly was just goading him into killing Duncan. She plays all hard-boiled but she comes to pieces scenes later while he's getting more and more bloodthirsty; and there's some implication he may have had her done to death.