Macbeth enters. She asks why he spends so much time alone. Macbeth responds: "We have scorched the snake, not killed it" (3.2.15). He fears someone might try to kill him as he killed Duncan, and seems envious of Duncan's "sleep" (3.2.25).
In order to keep power built by violence, more violence is always needed. Macbeth knew this would happen; he's caught in the vicious cycle of violence...
Lady Macbeth reminds him to be "bright and jovial" at the feast. Macbeth tells her to act the same. But then Macbeth moans, "O, full of scorpions is my mind, dear wife!" (3.2.37) because Banquo and Fleance are still alive.
...and that vicious cycle begins to take a psychological toll on Macbeth.
Macbeth says that before the night is through there shall be a "deed of dreadful note" (3.2.45), but adds that she's better off being innocent until she can applaud what has happened.
Macbeth tries to protect Lady Macbeth: traditional male-female roles.