Who is more evil, Lady Macbeth or Macbeth? How can you justify the answer?

At the beginning of the play, Lady Macbeth is more evil than Macbeth.  In fact, she fears that he is "too full o' th' milk of human kindness" to kill Duncan and take the shortcut to the throne (1.5.17).  She is almost right.  After they've made their plans to murder the king, Macbeth is so guilt-ridden and conscience-plagued that he hallucinates a dagger (their weapon of choice for the murder), has dinner with Duncan, and...

At the beginning of the play, Lady Macbeth is more evil than Macbeth.  In fact, she fears that he is "too full o' th' milk of human kindness" to kill Duncan and take the shortcut to the throne (1.5.17).  She is almost right.  After they've made their plans to murder the king, Macbeth is so guilt-ridden and conscience-plagued that he hallucinates a dagger (their weapon of choice for the murder), has dinner with Duncan, and then tells her, "We will proceed no further in this business" (1.7.34).  She, on the other hand, is praying for murderous spirits to "unsex" her and fill her with "direst cruelty" (1.5.28, 1.5.50).  She is ready and willing to murder Duncan and frame his guards without hesitation.

However, by the end of the play, Lady Macbeth is the one who is guilt-ridden, as we see in her sleepwalking scene (5.1).  She cries and tries to clean her hands of the blood that she, in her feverish dreams, can still see and smell there.  Further, the fact that she later commits suicide tells us how traumatized she's been by what she and Macbeth did, as well as what he has done since then.  In many ways, she's created a monster.  Without her knowledge or input, Macbeth has ordered the murders of Banquo and his son Fleance (though Fleance escapes) as well as Macduff's wife and children.  He has ruined Scotland with his greed and paranoia.  Therefore, Macbeth becomes the more truly evil in the end.