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Told by three witches he will be king, the Scottish general Macbeth, pushed by his wife, kills King Duncan while Duncan is a guest in his castle and takes the crown. To keep it, he has his friend Banquo murdered and orders the killing of Macduff's wife and children. He ends up isolated, his wife dead by apparent suicide, and is killed in battle by Macduff.
The witches never tell Macbeth to kill anyone. They only predict that he will be king, and he turns that prediction into a plan by acting to make it true. The prophecy's power comes from the listener's existing desire, which reads a neutral forecast as a permission or a command.
After the first murder, each new crime is committed to protect the gains of the last. Macbeth says he is 'in blood stepp'd in so far' that turning back is as hard as going on. The cost of stopping rises with every transgression, so violence that began as one calculated act becomes an ongoing, almost mechanical tyranny.
Lady Macbeth dismisses the blood with 'a little water clears us of this deed.' Later, the guilt she refused to face shows up as sleepwalking and compulsive hand-washing. The play treats the conscience as something that cannot be argued away. Guilt denied at the level of will comes back through involuntary behavior, hallucination and sleeplessness.