Answer:
Lady MacBeth lived in a male-dominated society. The only two ways a woman could move up in the world was 1) by marrying a man who was more wealthy or came from a higher social class or 2) if her husband moved up in the world. I imagine she believed that by convincing her husband to kill Duncan and thus becoming queen - she would have all the validation she ever wanted, and have more power/control in her life (something nearly all human being crave). Note, she also may have been thinking about how much better her children’s lives would be if they were royalty….of course that didn’t work out.