Gertrude
She did not merely marry a second time but was also unfaithful to her husband while he was alive; this is indubitable since the Ghost states so.
On the other Hand, the way Shakespeare sees it; she was not an accomplice to her husband's murder. This is evident in the closet scene where Hamlet cries:
A bloody deed! Almost as bad, good mother
As kill a king and marry with his brother
And she replies in genuine bewilderment: 'As kill a king!' More important evidence as to this is the fact that Claudius and her never talk about the murder of the king. …
