The main way in which Macbeth's character changes after the murder of Duncan is that, where once he was so hesitant to commit murder that his wife scorned him for being a weakling, he now appears to lose all scruples. He arranges for the murder of Banquo and Fleance without hesitation, and goes even further in planning to eliminate Macduff and his family. He attempts to get rid of anyone whom he fears might stand in his way. Of course he does not succeed; Fleance and Macduff escape, and Macduff finally conquers and kills him. But this shows that, once he begins killing, he will stop at nothing.
Therefore Macbeth might end up appearing as a pure villain but we also have a continuous insight into his mind and we see that he becomes practically unhinged over the killing of Duncan, beset by hallucinations before and after. Thereafter he just gets more and more worked up, attempting to shore up his ill-gotten kingship by any means. His actions become ever more murderous, it's true, but his motive appears to be desperation rather than bloodlust.