Although Macbeth is having qualms about killing King Duncan, knowing it means crossing a line that can never be crossed again, Lady Macbeth's ambition and relentless goading and pushing of him to do the act influences him to do what, in his heart of hearts, he knows is a terrible mistake. She uses language—rhetoric—about her own strength of mind and ability to harden herself and act like a man that causes Macbeth to feel he would be unmanly if he didn't go through with the murder. He can't let his wife be more manly than he is.
The witches also use language in ways that lead to Macbeth's demise. Most importantly, they lead him to believe the prophecy that he will become king of Scotland. Later, when they offer more prophecies, such as that he will not be defeated until Birnam woods move to Dunsinane and that he will not be defeated by any man born of woman, he also believes them.
In Lady Macbeth's case, her rhetoric is just that—words. In the end, she goes mad from guilt over the murders: she wasn't as tough as she thought. In the case of the witches, their words trick and trap Macbeth because they meant something different from the way Macbeth interpreted them: MacDuff was born by C-section, and the Birnam woods move when MacDuff's men cut branches to disguise themselves.