King Claudius of Hamlet verges upon being a stock villain: He is unconscionable, unethical, immoral, hypocritical, without remorse for many of his acts, Machiavellian in his rule. From the beginning of the play, his evil nature is pronounced: In Act I, the ghost of King Hamlet alludes to his brother as like Adam in The Fall:
[t]he serpent that did sting [Hamlet's] father's life
Now wears his crown (1.5.7-38-39)
and, further, alludes to the immorality of his brother,
...that that incestuous, that adulterate beast
With witchcraft of his wit, with traitorous gifts...
won to his shameful lust
The will of [King Hamlet's] most seeming-virtuous queen (1.5.41-46)
But, while Claudius possesses the "vaulting ambition" of Macbeth and plots the murders of those who impede his path to autocratic rule, he is not without remorse. For, after the "Dumb Show," in Act III, Claudius feels extremely guilty--
O, my offence is rank, it smells to heaven;
It hath the primal eldest curse upon't,
A brother's murder...(3.3.36-39)
--yet his desire for power overrides his regret as he enjoys his power,
...I am still possessed
Of those effects for which I did the murder--
My crown, mine own ambition, and my queen. (3.3.53-55)
In accord with Machiavelli's theory, an unconscionable man can be an effective ruler, and Claudius, who arrests the threat of invasion by Norway and halts Laertes in his desire to murder Hamlet outright, an act which would disrupt the Danes who love their prince, is the bad man who is a good king. Nevertheless, his flaws are his nemesis. For, "way leads to way" and Claudius accelerates his nefarious deeds as he feels he must eliminate those who would expose him. So, he manipulates Laertes, who returns to Denmark with the intent of avenging the death of his father and convinces him to duel Hamlet so that the death of the prince will not be an outrageous act, but one of self-defense. Proud of his skill to sway others, Claudius flatters Laertes's fencing skills and suggests that Hamlet will accept a challenge to duel because of his envy of Laertes's reputation, especially if he is convinced that the king has wagered upon the outcome of this duel. Laertes falls for Claudius's canard and vows to "anoint" his sword with poison so strong that it will kill Hamlet if he is but scratched by the lance. For insurance, Claudius promises to poison a drink to give Hamlet when he thirsts.
His mounting anxiety about Hamlet about whom he has observed,
There is something in his soul
O'er which his melancholy sets on brood...(3.1.64-65)
causes Claudius to become rasher in his behavior as he feels he must eliminate the prince, who has already escaped death in England and returns to Denmark, requesting an audience with Claudius "alone." The king's plan to use Laertes fails when Hamlet strikes Laertes and Gertrude drinks to her son's success; unfortunately, she drinks from the cup Claudius has poisoned. Somehow the duelers have picked up each other's swords, and when Hamlet strikes Laertes, the poison enters the wrong man. Laertes says, "I am justly killed with my own treachery" (5.2.286);then, Gertrude cries out, "O my dear Hamlet....I am poisoned!" (5.2.289), and Hamlet orders the doors locked after Laertes places the blame upon the king. Enraged, Hamlet wounds Claudius and forces him to drink the remaining poison in the cup intended for him. Thus, Claudius in his cupidity villainous ambition has wrought his own death.
This is purely my opinion: It seems to me Claudius's foremost flaw is his ego. In theory, if he had not been so power hungry and greedily lusted after Gertrude he would not have killed King Hamlet, and the entire plot would have been avoidable. Through the entire play not one of his actions is selfless; he is wholly self-seeking. Even in the end his obsession to get rid of Hamlet results in disastrous consequences and collateral damage for which he shows no remorse. Denmark is left in the hands of Prince Fortinbras because the entire elective monarchy is wiped out due to Claudius's foolish plans.