Lady Macbeth may be all-in for killing Duncan, but her husband's not so sure. Macbeth appears to be getting cold feet about the wicked murder plot, as can be seen in the intense debate he conducts with himself in his soliloquy in act 1, scene 7.

During his soliloquy, Macbeth makes a number of astute observations, all of which count against his going ahead with the killing of Duncan. Macbeth is Duncan's loyal servant and kinsman; instead of having him killed, he should be protecting him. And as Duncan's host, Macbeth should be shutting the door in Duncan's murderer's face himself, not carrying out the dirty deed.

But perhaps the biggest count against killing Duncan is that it will lead to what we might call bad karma, or what goes around comes around, as the saying goes. As Macbeth wisely reflects, justice, being equal to everyone, forces us to drink from the same poisoned cup or chalice that we serve to others. In other words, if we do wrong to someone, then that wrong will eventually come back to haunt us.

And that's precisely what will happen to both Macbeth and his wife. Lady Macbeth's participation in Duncan's murder causes her to develop powerful guilt complexes from which she cannot escape. Unable to live with her guilt, she commits suicide.

As for Macbeth, his murder of Duncan will set him on the path to absolute tyranny, which in turn will create numerous enemies determined to remove him from the Scottish throne by force. One such enemy is Macduff, whose family is murdered on Macbeth's orders and who gains revenge on Macbeth by killing him in a duel. This is Macbeth's payback for killing Duncan. He has been forced to drink from the poisoned cup of justice. Or, as he puts it himself:

This even-handed justice
Commends the ingredients of our poisoned chalice
To our own lips.
(act 1, scene 7, lines 10–12)