How does Macbeth betray Lady Macbeth?
He betrays Lady Macbeth in the sense that he dispenses her advice and starts to act unilaterally. Lady Macbeth, more than anyone else, is responsible for her husband's rise to power; she made him what he is. She planned King Duncan's murder; she questioned Macbeth's manhood, goading him into committing the wicked act; she even drugged Duncan's guards and planted bloody daggers on them to avoid the merest hint of suspicion. But as soon as Macbeth's on the throne, he starts acting on his own initiative. Hubris takes over Macbeth's character, dissolving what had previously been very much a power marriage. Puffed up with such overweening pride, Macbeth carries out a series of increasingly vicious murders, showing that whatever "milk of human kindness" there may have been in his soul has now turned rancid.
To some extent, Lady Macbeth has helped create a Frankenstein's monster which is now completely out of control. As the bodies start piling up, and Macbeth wades deeper and deeper through blood, Lady Macbeth cannot handle the immense psychological strain that her husband's murderous rampage has unleashed upon her fraying mind. Wicked though it was, at least Lady Macbeth's plot to murder Duncan had a certain rationale behind it. But there appears to be no such justification for Macbeth's murder of Macduff's family, for example. Crucially, Macbeth is defying fate, whereas Lady Macbeth's plot to murder Duncan was entirely in keeping with the Weird Sisters' prophecy. Macbeth, in defying fate, is also betraying his wife, leading directly to her psychological breakdown and eventual death.