How can I justify the main characters in Macbeth?
The degree to which the actions of the characters in William Shakespeare's Macbeth can be justified depends on the individual characters. You might take the following approaches:
King Duncan: As a legitimate king, he has the right, and even duty, to suppress a rebellion. While he treats the Thane of Cawdor harshly, he is acting within his right as king and attempting to preserve and defend his kingdom. Legally, he has the right to declare Macbeth Thane of Cawdor as well.
Three Witches: Although witchcraft was illegal, one could argue that it was one of the few paths to power for women in a patriarchal society. The witches don't actually commit murder or even compel Macbeth to act; he makes those decsions on his own. They merely provide information that proves to be accurate.
Macbeth: His actions are very difficult to justify as he killed a kind and generous king to advance his own grasp of wealth and power and was a harsh and evil ruler. One could argue that politics in that period was harsh, or that he was persuaded against his better judgement by Lady Macbeth and the witches.
Lady Macbeth: In this period, a loyal wife was supposed to help and support her husband. She must steel herself and strengthen her husband's resolve to do what is best for her family, as she states:
... Yet do I fear thy [Macbeth's] nature;
It is too full o' th' milk of human kindness
To catch the nearest way. ...
Macduff and Malcolm do rebel against Macbeth, but they are justified by Macbeth's murder of Duncan and his tyrannical mode of rulership.