What's the theme of Hamlet?
There is surely more than one theme to a play as complex as Hamlet is.
On one level, the play is about corruption, evil, and the need to take revenge. We see this as Hamlet ends up killing all those he feels are responsible for the killing of his father.
On another level, this is a play about death and the futility of life. You can see the idea of the futility of life in the famous "to be or not to be" soliloquy. You can see the theme of death in the fact that essentially all the major characters die.
There are surely other themes that others would see in this complex play.
Like many of Shakespeare's plays, Hamlet can be read and interpreted on many different levels. The most popular and common theme, however, seems to be the theme of revenge. The audience sees this theme play out in the tragedy as Hamlet vows to avenge his father's death. He becomes so consumed in his quest that he ends up destroying the relationships (and, in some cases, lives) of those he loves, while he also begins questioning death and his own mortality. I would say this is also a central theme because everything else plays off of Hamlet's obsession with seeking revenge.