Why was Lady Macbeth killed?
Most scholars believe that Lady Macbeth kills herself, as Malcolm's closing speech in Act 5, Scene 8 suggests this fact: "...his fiend-like queen, / Who, as 'tis thought, by self and violent hands / Took off her life" (69-71). Using this speech as their guide, most scholars hypothesize that Lady Macbeth was not killed by a person, but committed suicide. This idea logically follows from Lady Macbeth's increasingly maddened actions: as the play goes on, Shakespeare shows us a queen driven mad by her vile crimes. As such, it's possible to assume that, crazy with grief and guilt, Lady Macbeth commits suicide in the final scenes of the play.
That said, Shakespeare loves to employ ambiguity in his dramatic work, and so it's fitting that a great deal of ambiguity surrounds the death of Lady Macbeth. She dies offstage, and so we don't know for sure exactly how she dies. Malcolm merely reports a very plausible rumor concerning her death, but he does not know the truth for sure. As such, it's theoretically possible (although not overtly supported within the text itself) that Lady Macbeth was killed, potentially by a member of the court who resented the queen's evil actions. Though it's much more likely that Malcolm's theory is correct, the ambiguous nature of Lady Macbeth's death allows for the possibility of foul play.