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In Shakespeare's time the only way for women to gain power was through their husbands. Lady MacBeth asks the spirits to make her evil and then she proceeds on to convince MacBeth to kill Duncan. If MacBeth becomes king then Lady MacBeth will become the queen. The opportunity presented itself and Lady MacBeth wanted her husband to act on it. She may be more greedy than ambitious however, because she wanted the fame without having to do it herself. Ambition has more to do with action while greed has more to do with desire.