How is the theme of good vs evil depicted by Shakespeare in his play, Macbeth.

Evil in Macbeth is best illustrated by the actions of the the three witches, Macbeth, Lady Macbeth, the assassins hired by Macbeth later to commit his evil, the king of Norway, and the traitors, Macdonwald and the thane of Cawdor.

In contrast to this, good is best depicted by King Duncan, his older son, Malcolm - rightful heir to the throne, Banquo, Macduff, Lady Macduff and the king of England, Edward, as well as the forces who are gathered to overthrow the usurper and tyrant, Macbeth.

It is ironic that Macbeth, who virtually becomes the epitome of evil, is initially depicted as good and honorable. An injured sergeant's report of him at the beginning of the play speaks of a courageous general who was prepared to sacrifice his life for king and country, as illustrated in the following extract:

For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage...

It is his 'overriding ambition' as well as the witches' predictions and his wife's urging, that turns Macbeth into a bloodthirsty tyrant. He and his wife plot the king's assassination and then murder him in his sleep, whilst Macbeth also kills the king's guards to prevent any suspicion falling on him. This act sets him off on a pernicious journey of vile evil. Once he is crowned, he becomes paranoid and sets out to destroy whomever he deems a threat. In the process, he has his best friend, Banquo, murdered and sends out assassins to kill Macduff's entire family.

Lady Macbeth, ironically, appears to be the more evil of the two partners in crime at the beginning. She urges her husband to carry through their evil plot when he expresses doubt. The depth of her perfidy is best illustrated by the following quote from Act 1, scene 3:

I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.

In the end, it is she who is destroyed by remorse for what they have done. She iss tortured by visions of their crime and, overwhelmed by guilt, she commits suicide whilst Macbeth heads toward his own doom, believing that he is invincible since the witches predicted that 'no man of women born shall harm Macbeth.' The evil tyrant is eventually killed in a fight by Macduff, who had been 'from his mother's womb untimely ripped.'

The witches take great pleasure in watching Macbeth's road to ruin. They set out to deliberately confuse him, using paradox and equivocation, literally leading him down the garden path. Their predictions encourage and empower him to commit further and greater evils.

The king of Norway, Macdonwald and the thane of Cawdor represent evil because they all plotted against king Duncan, planning to usurp his throne. They were defeated by the forces of good, ironically lead by Macbeth at the time, who had obviously been assisted mainly by his courageous compatriots, Banquo and Macduff. 

The remaining forces of good, (i.e. after Duncan's assassination) represented by the characters named previously, initially suffer heavily at the tyrant's hands. Banquo, who has been suspicious of Macbeth from the outset, is murdered but his son, Fleance, manages to escape.

Macduff, who is aware of Macbeth's malice, flees to England where he seeks assistance from the English king and joins forces with Malcolm to plot the tyrant's defeat. It is a necessary and desperate move and he pays dearly for it, for Macbeth has his entire family murdered. This, however, encourages Macduff to seek vengeance and ensure Macbeth's annihilation, as depicted in his passionate declaration in Act 4, scene 3:

But, gentle heavens,
Cut short all intermission; front to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!

Edward, the king of England, who has taken Malcolm under his wing, is depicted as one blessed with healing powers. In effect, the implication is that he will provide the cure for Scotland's disease - Macbeth. He vows to assist Malcolm by providing ten thousand English troops to invade Scotland.

In spite of their earlier setbacks, those who represent good, under the leadership of Malcolm, his uncle Siward, and Macduff, soldier on and eventually defeat the tyrant Macbeth, whose decapitated head is displayed by Macduff for all to see. Malcolm becomes the new king of Scotland. The forces of good have triumphed.