Hamlet is convinced that she has committed a grevious sin by covering up Claudius' horrific deed and, perhaps even worse, continuing to live with the man and share his bed. He begs his mother to repent and reform her ways, saying, "Virtue itself of vice must pardon beg,/Yea, curb and woo for leave to do him good" (4.3.175-176).
Gertrude, however, is not willing to leave Claudius. Instead, she wrings her hands, wanting to stay with her lover and yet not see her son in such pain: "O Hamlet," she laments, "thou hast cleft my heart in twain!"..."What shall I do?" (4.3.177,201)
Hamlet tells her in no uncertain terms what to do:
"Not this by no means that I did you do:
Let the bloat king tempt you again to bed,
Pinch wanton on your cheek and call you his mouse,
Or paddling in your neck with his damned fingers,
Make you to ravel all this matter out" (4.3.203-209).
Gertrude gives him no answer that pleases Hamlet in 4.2. She is, in Hamlet's judgment, is just as complicit in the death of his father as is Claudius by her contiuning unrepentance.