King Hamlet's ghost introduces himself in a way that most certainly evoked the sympathy of the Elizabethan audience. He tells Hamlet that his brother robbed him of everything he was, all that he owned, including his everlasting soul. In the same way that the Bible engenders sympathy for Abel and condemns Cain for the fratricide, Shakespeare favors the murdered brother.
Hamlet is quick to believe the Ghost because the spirit's words confirm his worst fear: Claudius murdered King Hamlet. For the Elizabethan/Jacobean audience who attended the first performances of Hamlet, murder of a king was in itself cause for alarm. Consider that the English people believed that their monarchs ruled by Divine Right, that God Himself appointed them to rule the land. The Church of England went so far as to attribute to the monarch the highest order of executive power in the church as well. In all ways, the English monarch represented God on earth. King Hamlet's murder makes the Ghost a most sympathetic figure to Shakespeare's audiences. No one would have questioned the existence of that Ghost, and few would have believed — even for a moment, as Hamlet does — that the Ghost could be a devil.
The fact that his mother's lover is also her husband's murderer exacerbates Gertrude's crime of incest. Hamlet is bereft of choice. He may have an aversion to violence, and he may live by strict Christian principles, but he must avenge his father's honor. Hamlet sees no way to honor his father except by killing Claudius. Doubly impelled by his father's orders and by tradition, Hamlet becomes a prisoner of his obligation for revenge.
