This paper examines Hamlet's quest for vengeance by looking at the text directly and taking cues from secondary sources. It concludes that Hamlet's delay in taking vengeance is primarily based on his indecision, his self-interest, and his desire to protect Gertrude.
From the Paper:
"Shakespeare's Hamlet is a play about revenge and indecision. Because Hamlet's father is wronged, Hamlet himself spends most of the play deliberating on the subject of his revenge against his uncle Claudius, who has killed his father, married his mother, and assumed the throne. Hamlet's adherence to the patriarchal ideals is supported throughout the play by the appearance of his father's ghost, which appears as a sort of dream-like interruption of the quotidian reality in which Hamlet might deny the filial duty to avenge his father. This ghost spurs the prince on to seek revenge, even when Hamlet seemingly wants to get on with his life and save his soul in the process."