I'll just add a bit to mstultz72's excellent answer above, concerning Shakespeare's Macbeth.
Lady Macbeth's power, in this situation, comes from the fact that she does not possess the scruples that her husband does. When he has second thoughts about killing Duncan, she will not. She is more ruthless. She cares less for the possible consequences. She never mentions anything about the possible spiritual consequences in the afterlife, as Macbeth does, for instance. And she does not care that Duncan has treated Macbeth well, or that others in the country have recently developed a respect for and liking of Macbeth, as Macbeth does. She will not change her mind about killing Duncan. This gives her an evil power over her husband and, as it turns out, Scotland.
Of course, while her lack of scruples and concern for consequences definitely leads to her husband's assassination of Duncan, she is not so resistant to "good" as she first appears. She is unable to kill Duncan when she has the chance (because the sleeping Duncan reminds her of her father), and ultimately commits suicide as a result of her feelings of guilt.