Macbeth murdered his best friend, Banquo, for two different reasons. The irony here is that Macbeth, the new Thane will be even more treacherous.
Unfortunately though her abilities are dirty and evil. Shakespeare through the characters of Macbeth and Lady Macbeth demonstrates that appearances can be deceiving. This is perhaps the line that lays siege to Macbeths mind, and the start of a downward spiral of mental deterioration.
I personally do not think Macbeth is evil until after he murders Duncan. We learn also that Lady Macbeth could not herself kill Duncan, as he reminded her of her father as he slept.
At Macbeths banquet the murderers arrive and tell Macbeth that Banquo was killed but Fleance escaped. He is now a tyrant, ruling with fear. This is the mastery of Shakespeare? Macbeth is a good man who is troubled by his conscience and loyalty though at the same time ambitious and murderous.
This makes him want to get rid of Banquo and his only son so that he might have a better chance of his children becoming kings after him. By this stage her character has changed from being a ruthless, cruel and evil person to being slightly 'innocent' losing through her madness the evil in her.
Here Macbeth's violence and killings are praised because they have preserved the rightful king. However, it was Lady Macbeth's insistence on Duncan's death that started Macbeth's corruption and led to him being a tyrant because alone Macbeth would not have acted as he harboured too many doubts.