I would argue that Lady Macbeth's suicide is further and final indication of her "greater capacity for evil". In this act, she abandons her husband to deal with his crime and his guilt alone.
She pushed him on to the crime then left him holding the bag.
It is ironic indeed that Lady Macbeth proves incapable of living with their evil deeds than Macbeth since in Act I she has ridiculed her husband for his unmanliness as he reconsiders his decision to assassinate King Duncan. Seeming coldly cruel, she tells Macbeth that had she sworn to an act, she would even kill her suckling babe if necessary. Yet, she is mentally weaker than her husband who travels his bloody path until he himself is killed.
I also believe that while Lady Macbeth seems more evil initially, Macbeth does possess the greater capacity for evil. Macbeth, unlike Lady Macbeth, excludes her from his plans ("Be innocent of the knowledge, dearest chuck."). Lady Macbeth needs her husband to "be evil." Macbeth, on the other hand, does not need his wife.
I would argue that although Lady Macbeth starts off as being greater in terms of her capacity for evil, very quickly in Act III we see Macbeth showing his own inner potential for evil as he begins to plot his own murders without consulting with his wife and working more and more independently. In addition, I think we can see a definite escalation of the kind of crimes he commits. He moves from killing Duncan with his own hands to employing others to kill to finally slaughtering whole households.
Initially, at least, Lady Macbeth seems the more evil of the two -- so evil that her husband seems shocked at the arguments she uses when she tries to convince him to commit the murders. Later, though, she does seem to feel very guilty -- so much so that she is basically absent from the very end of the play.
Some critics might say Macbeth has a greater capacity for evil than Lady MacBeth because when he reaches the point of no return, we see him question himslef and his plans. He considers his position, his options and examines his 'conscience' (if he has one, that too is permanently up for debate!) and then decides to continue his evil path. Either William Shakespeare is trying to show us that MacBeth is fully aware of his 'sin' or 'evil-doing' and is therefore culpable - or that he is mad. The jury is still out.