I think Lady Macbeth is responsible for the murder of King Duncan, however she is not ultimately more responsible than Macbeth. Both Lady Macbeth and Macbeth are 'evil' characters. Lady Macbeth is 'evil' in the sense that she is aware that what she is doing is immoral, however she does so to benefit herself--obtaining the position of Queen. Macbeth is 'evil' in the sense that he is very dependent on his wife, thus adopting her evil mindset, and also because he values his vaulting ambition above his morality. Examples of this is him killing King Duncan, Banquo and MacDuff's family.
So, while both characters are evil, I think that Macbeth is ultimately more responsible for the death of King Duncan. In a literal sense, Macbeth is the one who plunges the knife into King Duncan and stages the crime scene (blaming the guards). Lady Macbeth is manipulative over her husband, as she was pressuring him to murder King Duncan, however Macbeth was not totally against the idea of murder--as when the play progresses, he plots Banquo's murder without command from Lady Macbeth.
