As matters stand, the truth can be brought to light only by the royal criminal's betraying himself. Hence Hamlet's perfectly logical, most ingenious device for forcing him to do so. Hamlet's object is not to take a purely material revenge for the crime, but to reinstate right and justice in Denmark, to be judge and avenger in one. And this he cannot be if he simply kills the king off-hand.
All this is acute, and in part correct; only it misstates the theme of the play. Had Shakespeare had this outward difficulty in mind, he would have made Hamlet expound, or at least allude to it. As a matter of fact, Hamlet does nothing of the sort. On the contrary, he upbraids himself for his inaction and sloth, thereby indicating clearly enough that the great fundamental difficulty is an inward one, and that the real scene of the tragedy lies in the hero's soul.
Hamlet himself is comparatively planless, but, as Goethe has profoundly remarked, the play is not therefore without a plan. And where Hamlet is most hesitating, where he tries to palliate his planlessness, there the plan speaks loudest and clearest. Where, for example, Hamlet comes upon the King at his prayers, and will not kill him, because he is not to die "in the purging of his soul" but revelling in sinful debauch, we hear Shakespeare's general idea in the words which, in the mouth of the hero, sound like an evasion. Shakespeare, not Hamlet, reserves the King for the death which in fact overtakes him just as he has poisoned Laertes's blade, seasoned "a chalice" for Hamlet, out of cowardice allowed the Queen to drain it, and been the efficient cause of both Laertes's and Hamlet's fatal wounds. Hamlet thus actually attains his declared object in allowing the King to live.
[1] K. Werder: Vorlesungen über Hamlet, p. 3 et seq.
[2] Georg Brandes: Æsthetiske Studier. Essay "On the Concept: Tragic Fate."
[3] Karl Werder: Vorlesungen über Shakespeare's Hamlet, 1875.