In Act 3, scene 4, how does Gertrude defend herself or how does she react to Hamlet's accusations and his action?
When Hamlet murders Polonius, Gertrude continually questions him. She is distraught by what he has done and seeks to make sense of his rants. She asks:
O me, what hast thou done?
As kill a king?
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?
It is clear by the repetition of "oh me" & "ay me" that she is somewhat in a panic, confused by her son who has calmly killed a man & continues to attempt to convince her of her own sin. As Hamlet accuses her of incest, telling her that she lives "in the rank sweat of an enseamed bed/Stew'd in corruption, honeying and making love/Over the nasty sty!", Gertrude grows increasingly desperate, pleading with her son to stop.
O Hamlet, speak no more!
Thou turn'st mine eyes into my very soul,
And there I see such black and grained spots
As will not leave their tinct.
O, speak to me no more!
These words like daggers enter in mine ears.
No more, sweet Hamlet!
She seems to be agreeing with his assessment of the situation, referring to the "black and grained spots" on her soul. She does think he is mad, but he repeatedly insists on his own sanity. His final command to her is to keep the secret of his sanity, & let everyone else continue to think he's crazy. She agrees, but does she believe him? Does she keep that secret? There is evidence in the text to support either interpretation.