Just to play devil's advocate here for fun, I want you to consider an important part of Hamlet that shows Claudius might not be 100% evil. Remember, Hamlet catches Claudius praying for forgiveness in church! (As a result, Hamlet again commits the tragic flaw of inaction by not avenging his father at that moment.) First, the evidence of Claudius' repentance:
O, my offense is rank, it smells to heaven; / It hath the primal eldest curse upon't, / A brother's murder. ... Is there not rain enough in the sweet heavens / To wash it white as snow? Whereto serves mercy but to confront the visage of offense? ... My fault is past. But, O, what form of prayer / Can serve my turn?
So, Claudius realizes here that he is, in fact, sorry for what he has done. His issue is that he still has the "effects" of his sin: Gertrude. This monologue is a testimony to that caveat. Does this prove that Claudius is not evil in nature? Hmmm, not really, but it does show that he does have a conscience and that conscience is causing him trouble.
And now let's look at those lines where Hamlet overthinks this:
Now might I do it pat, now 'a is a-praying, / And now I'll do't. And so 'a goes to heaven, / And so am I revenged. That would be scanned. / A villain kills my father, and for that / I, his sole son, do this same villain send to heaven.
Poor Hamlet. Poor, intelligent Hamlet. He just can't get this thing done, can he? In regards to your question, though, it's the reasoning here that's important. Hamlet himself admits that Claudius is most likely asking for true repentance here and, because Hamlet doesn't want to send his arch enemy to heaven for that true repentance, Hamlet doesn't do the deed. Hamlet would much rather see Claudius rot in hell.
So who is the real evil character here? The one asking for repentance, ... or the one hoping his enemy rots in hell? Hmmmmm, ...
Like I said, ... devil's adovcate. : )
Claudius is an evil character who has little remorse for killing his brother and marrying his brother's wife.
When Hamlet reenacts his father's murder, Claudius realizes that Hamlet suspects him of the murder. What does he do? He prepares to have Hamlet murdered.
Rather than changing his evil nature, he continues to plot murder.
Although he does repent for murdering his brother, he turns around and plots the murder of Hamlet.
He has not changed. He is murderer.
In the end, Hamlet avenges his father's death when he kills Claudius, but it has costs Hamlet his life as well.