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Simply put, she wanted to be Queen. One of the larger themes of Macbeth is lust for power. L. Macbeth's only way to get this power was by her husband becoming king. L. Macbeth, of course, felt she would be the real power behind the throne.
Her motives were ambition, and love. Her union with Macbeth was very much a partnership in the beginning -in his letter to her cataloguing his meeting with the witches he refers to her as his 'dearest partner of greatness'. She wishes that as a couple they accede to the throne. Her involvement in the murders is largely due to the doubt she has in her husband's resolve to commit so great a sin as killing a king. She states that he is 'too full 'o' the milk of human kindness' to be a truly ruthless killer.