First of all, Lady Macbeth notices that her husband is disturbed. She wonders why he has been keeping to himself and worrying over the past. She tries to cheer him up with
Things without all remedy
Shoud be without regard; what's done is done.
Macbeth replies, though, that he is not happy, and uses a metaphor declaring
We have scotched the snake, not killed it.
Killing Duncan has not brought an end to Macbeth's worries. He is worried because Banquo still lives. He admits that he has not slept well, that "terrible dreams" have disturbed his sleep, and that his mind is "full of scorpions."
Guilt and fear plague Macbeth, so he focuses on Banquo. He thinks killing Banquo will give him some relief from his fears of being found out and fears that he killed Duncan in vain.