What is the full quotation about "bad mother" in Hamlet?
Your question apparently refers to the exchange between Hamlet and his mother in Act III, Scene 4, right after Hamlet kills Polonius, who was hiding behind the arras. Gertrude is horrified. She says,
O, what a rash and bloody deed is this!
A bloody deed. Almost as bad, good mother,
As kill a king, and marry with his brother.
Hamlet is testing Gertrude. He wants to see how she reacts, whether she will give herself away with a guilty expression or even confess. He still suspects she was somehow involved in his father's murder. She may not have been an accomplice, but she may have known either that Claudius intended to commit the crime or else have known he was guilty of committing the crime after her husband's dead body was discovered in the garden. Gertrude's reaction seems to indicate she is innocent of any kind of complicity in the murder. She simply asks,
As kill a king?
Gertrude doesn't understand. She still believes her son is insane. Her further utterances suggest she is innocent of any wrongdoing except for marrying Claudius without observing a decent period of mourning for her dead husband and perhaps for what Hamlet considers an incestuous marriage. Here are two examples of her dialogue which suggest her innocence of any complicity in her late husband's death:
What have I done, that thou darest wag thy tongue
In noise so rude against me?
Ay me, what act,
That roars so loud and thunders in the index?
Hamlet must be convinced. Throughout the scene, he focuses on his mother's guilt in marrying an inferior man like Claudius and doing it so precipitously. He reduces his mother to tears. He will not attempt to convince her that Claudius killed her former husband, and he does not attempt to get her to conspire with him against Claudius. She will remain loyal to her new husband, as shown when Laertes storms into the castle at the head of a mob in Act IV, Scene 5 and threatens to start a rebellion. She will keep Hamlet's secret that he is not mad, though, and it is possible that she keeps her promise not to engage in any more lovemaking with Claudius.