Initially, Gertrude hopes to discover the cause of Hamlet's bizarre behavior as Polonius eavesdrops behind a tapestry in her room. When Hamlet enters Gertrude's chamber, he accosts his mother, and she responds by challenging her son's negative views of her. Hamlet then hears a noise from behind the tapestry and unknowingly murders Polonius, thinking that Claudius is behind the tapestry. Following Polonius's murder, Gertrude says that Hamlet has conducted such a senseless, horrible act. Hamlet responds by saying,
A bloody deed? Almost as bad, good mother, As kill a king and marry with his brother. (Shakespeare, 3.4.29-30)
Gertrude reveals her innocence by asking Hamlet,
As kill a king? (Shakespeare, 3.4.31)
She is clearly unaware of what Hamlet is talking about and does not immediately recognize that Hamlet is referring to how Claudius murdered his brother. If Gertrude were complicit in her husband's assassination, she would not have questioned Hamlet and fully understood why he is upset with her.
(The entire section contains 3 answers and 666 words.)