Relativity means the absence of absolute or universal standards. Hamlet struggles with this problem throughout most of the play. Two ethical systems, both of which claim preeminence, collide for him. The first is the revenge system. A vestige of a pre-Christian culture, the revenge ethic requires that a son avenge the murder of his father. Hamlet is confronted with enacting this revenge when his father's ghost, wandering and suffering in Purgatory, demands point blank that he kill Claudius.
The revenge ethic collides with the Christian ethic that Hamlet has also internalized, which preaches forgiveness of wrongs and dependence on God for ultimate justice.
The problem posed by the clash of these two systems, neither one of which appears wholly absolute or authoritative to Hamlet, puts him into a state of anguish. Does he follow his father's orders? Is the ghost really his father or an illusion sent by Satan to tempt him to sin? What about his deep disinclination to kill his uncle? The inability to decide what is the right path leads Hamlet through the early part of the play to think about suicide: he was already thinking about it before he met with the ghost, but now it seems even more of an attractive way out of his moral dilemma. Yet he fears the Christian universe might be authoritative, and as he doesn't want to face the afterlife it dangles in front of a suicide, he doesn't act.
Hamlet ultimately comes to trust in a Christian God's guidance after what he considers God's providential salvation of him from the death decree carried by Rosencrantz and Guildenstern. At this point, he can place himself in God's hands with confidence. Before that point, however, he is buffeted between two ethical systems that are in opposition.