Why would Shakespeare include the theme of corruption and decay in Hamlet?

Corruption and decay are a constant theme in Hamlet. From Horatio's assertion early in the play that something is "rotten in the state of Denmark," to Hamlet's disturbing joke that "politic worms" are eating the corpse of the recently dead Polonius, to his reflections on viewing Yorick's skull in the graveyard, the characters seem obsessed with the subject. One reason why Shakespeare may have included this theme is that he wished to underscore the foulness of the crime Claudius had committed. By murdering his brother and marrying his former sister-in-law, the king has eaten away at the moral center of Denmark. Something is rotten indeed in Denmark, and Hamlet is alone (with only Horatio as a confidant) in his knowledge of exactly what the source of the corruption really is. The theme of corruption and disease also serves to emphasize Hamlet's apparent madness, a major manifestation of which is his obsession with death and mortality. This, of course, is the focus of his speech at Yorick's grave. Thus corruption and decay can be found at the heart of Denmark and in Hamlet's own psyche. Shakespeare drives home the ubiquity of this theme through evoking it in one scene after another.
http://www.shakespeare-online.com/plays/hamlet/diseaseimageryhamlet.html