Discuss how Hamlet is a commentary on the presence of evil in the world.

This is quite a broad topic. Evil as presented in Hamlet usually takes on the form of moral corruption resulting from earthly desires or passions, such as political gain, emotional rage, or sexual lust. Characters let their baser natures get the best of them time and again throughout the play.
For example, Hamlet decries the corruption of the Danish court, from his mother's sexual desire for Claudius (her former brother-in-law) to the betrayal of Rosencrantz and Guildenstern in spying on him for the king. Claudius is a man well aware of how evil he is, having murdered his brother for political gain, though he cannot bring himself to sincerely repent. Polonius is driven by political ambition as well, though in a more comedic manner. Hamlet also warns the innocent Ophelia to never marry, so that she will never have children or bring more "sinners" into the world.
So, evil in the world of Hamlet it mainly stays in the realm of the political and the moral. All of the characters appear to suffer some degree of corruption—even Hamlet himself, whose rash temper leads to the murder of people innocent of Claudius's wrongdoings. Ophelia, Rosencrantz, and Guildenstern are about as close to unwitting innocents as the play gets, and they suffer horribly too.