Macbeth in Shakespeare's Macbeth is the most evil character in the play.
He is the only major character who actually kills anyone for evil purposes. The witches introduce the idea of being king to Macbeth, but he takes that idea and turns it into another idea--assassinate Duncan to get the throne as soon as possible. And he does it.
Lady Macbeth manipulates her husband and talks him into going through with Duncan's assassination, but she doesn't actually kill anyone. And soon after Duncan's assassination, Macbeth shuts her out of the decision-making process and she has nothing to do with the rest of the killings in the play, except for the guilt she becomes obsessed with for having helped in getting it all started.
Macbeth not only kills Duncan, but orders the killings of Banquo and his son, and Macduff's family, including Macduff's son. He is the most evil character in the play.
I'll let another editor answer the second part of your question.
You should get some interesting discussion on this particular point. I think that it will be a challenge to argue that Macbeth is not the most evil character of the play. Certainly, he does not start out that way, but is rather created through other forces. Yet, he does an excellent job of running with these forces and plummeting into a moral abyss and a quagmire that is devoid of moral sensibility. I think that his devolution of moral and ethical restraint becomes the purpose of the play. It is in this light that one is able to see the relevance of the work when the reader examines how evil, if left unchecked and without opposition, can become all consuming where any demarcation of right or wrong is quickly eviscerated.
I would argue that Macbeth is the most evil of the characters. I do not think he is 100% evil. Especially at the beginning of the play, he seems to be normal.
However, as the play goes on, he gets to be so bloodthirsty and vicious that it is hard to think who could be more evil than he. His decision to kill Macduff's family, in particular, makes him seem evil to me.
To me, the play is relevant because of what it tells us about ambition and about guilt. These are two forces that really push Macbeth and Lady Macbeth to act as they do.
I must agree with the previous posters and stress that Macbeth is undoubtedly the most evil character in the play, even the witches acknowledge this aspect - "by the pricking of my thumbs, something wicked this way comes". It's a whole discussion regarding the predetermination of Macbeth's acts but it's obvious that the witches could influence the course of action with their predictions only by finding a "soil" to plant the evilness.
There is an interesting battle between wickedness and weakedness in Macbeth's character, the latter being what separates Macbeth from great Shakespearean villains, nevertheless, he is the most evil character of the play.
As for your second question, I am sorry but I don't quite understand it. The relevance of the play on what grounds? Macbeth is considered one of the four great tragedies, alongside with Hamlet, King Lear and Othello in various approaches of Shakespearean studies. Of course, there are variants in which the Roman plays are also added to the category, but no matter the interpretation, Macbeth is still one of the major Shakespearean works. If you wanted something different, you should clarify your inquiry.