Guilty about killing duncan and turning Macbeth into a killer. So she chooses to kill herself. Before this when she was sleepwalking in the beginning of Act 5 she confessed her crime and guilt to a doctor and a Gentlewoman by accident . “Out, damned spot! Out, I say!—One, two. Why, then, ’tis time to do ’t. Hell is murky!—Fie, my lord, fie! A soldier, and afeard? What need we fear who knows it, when none can call our power to account?( V.i.36-39). In this quote when Lady Macbeth is sleepwalking she is panicking because she can’t wash Duncan's blood off of her hands, and she can’t stop feeling guilty about what she has done. She forced macbeth to kill Duncan, this is why she chooses to kill herself. This proves why lady Macbeth feels guilty for turning Macbeth into a killer and helping kill Duncan.
In conclusion Lady Macbeth is mentally unstable. She longs for power and to become queen. But in the end she cannot handle pressure of murder and guilt on her shoulders. This is why she killed herself. The irony of it is if she didn’t guilt Macbeth into killing Duncan for not taking his chance and being a coward, she wouldn’t have felt guilty about anything at all. Because Duncan would still be alive and none of this would have ever happened. And maybe Lady Macbeth would end up sane in the end if it wasn’t for what she had