Hamlet never really hated Claudius before the murder, but what set of his hate towards him was not only the fact that he murdered his father, but the fact that he married his mother afterwards. He was disgusted by this incestual marriage between Claudius and his mother. Again his hate was brought to the surface by the ghost. He was told that he was to focus on Claudius and stop brooding on Gertrude. Hamlet's revenge comes with great cost to all. His previous inability to act and take revenge sooner caused the death of both his mother (who drank the poison that was meant for Hamlet) and Ophelia (who committed suicide because of Hamlet's murder of her father Polonius). This is ironic because by finally taking his revenge, he has destroyed the family whose honor he wanted to avenge. .
"One reason why it's Gertrude rather than Claudius who drives Hamlet up the wall is her total unconsciousness of having done anything wrong." (P88 Fry, Northrop, the Anatomy of Criticism) He knows that Claudius killed his father and is upset that his mother married him. Obviously she is unaware of what Claudius has done but hamlet does not feel that she should have married him so fast in the first place. "Because of her compliant nature, Hamlet finds her delightfully easy to bully, and she keeps crumpling under his ranting until the exasperated ghost comes in to derail him again." (P87 Fry, Northrop, the Anatomy of Criticism) .
Gertrude is a victim of circumstance. She is not completely guiltless, but there isn't any evidence that there is any desire in her to do anything bad. She is aware that her actions, her marriage to Claudius, may be the cause of Hamlets apparent madness. Hamlet was very upset at his mother for remarrying Claudius, and to deal with this he decided to be a little harder on her. But only after Hamlet tells her about how his father died. She doesn't take any steps to get away from Claudius.
