Two salient character traits of King Claudius are his qualities of being manipulative and corrupt.
Whereas his brother King Hamlet was a valiant warrior and just man, Claudius's designs are all suited to his desires. With the tool of his glib and manipulative words, Claudius holds power over others. His skillful use of language is demonstrated in Act I as he speaks to Hamlet. While his apparent intention is to console Hamlet, he really desires to manipulate him as well as his sister-in-law Gertrude, who has now become his wife and queen.
Very glibly, Claudius compliments, then lightly scolds Hamlet for grieving so long, telling him it is spiritually wrong,
'Tis sweet and commendable in your nature, Hamlet
To give these mourning duties to your father,
But you must know your father lost a father....
....But to persevere
In obstinate condolement is a course
Of impious stubbornness. 'Tis unmanly grief
It shows a will most incorrect to heaven. (1.2.87-95)
Further, Claudius urges Hamlet to think of him as his father now; he also plays to Gertrude and asks Hamlet to remain and not return to school in Wittenberg so that he can give "cheer and comfort" to his mother. Obviously, Claudius wants to impress Gertrude with his caring nature as he tries to keep her son near her to comfort her; however, his real intent in keeping Hamlet in Denmark is a sinister one.
Certainly, Claudius manipulates Laertes into dueling with Hamlet. In speaking more briefly to Laertes than he does to Hamlet, Claudius prevents Laertes from having time to consider his words, and therefore, to become more reactive. This shorter speech is less obscure than the longer one, and its directness calls for action on the part of Laertes, who responds accordingly, especially when Claudius alludes to inaction as a failure to demonstrate love for his father, thus challenging his filial love and honor.
Not that I think you did not love your father,
But that I know love is begun by time,
And that I see in passages of proof (4.7.108-110).
From the onset it is apparent that Claudius epitomizes what "is rotten in Denmark." When the ghost of his father talks to Hamlet, for example, he refers to Claudius as "that incestuous, that adulterate beast" (1.5.42). Claudius has committed fratricide and has married his brother's wife in a seemingly incestuous arrangement. Certainly, too, Claudius and his corrupt court delight in carnal pleasures:
The king doth wake tonight and takes his rouse,
Keeps wassail, and the swaggering up-spring reels;
And as he drains his draughts of Renish down,
The kettle-drum and trumpet thus bray out
The triumph of his pledge (1.4.8-12)
Claudius exploits everyone and acts unscrupulously. He manipulates Polonius into having Ophelia talk with Hamlet while having Hamlet's old friends Rosencrantz and Guildenstern spy on Hamlet. In addition, he cleverly motivates Laertes to kill Hamlet.
In Act V, he does not even tell Gertrude that the cup from which she drinks contains poison, lest blame fall upon him, merely urging her not to drink when she picks it up: "Gertrude, do not drink" (5.2.267). Later, in his declaration of revenge for his mother, before he forces Claudius to drink the poison, Hamlet reiterates the words of his father's ghost,
Here, thou incestuous, murd'rous, damned Dane,
Drink off this potion. Is thy union here?
Follow my mother. (5.2.304-306)
Manipulative and corrupt, King Claudius becomes a victim of his own evil as he swallows his poison.