When Macbeth murders Duncan, he can no longer go back to the point when his conscience was more or less clear. He has stepped into the realm of darkness, where he is tortured by his thoughts of insecurity and doubt. The witches' prophecy becomes engraved in his mind, and he cannot help but remember the witches said Banquo's sons would inherit the throne one day. This news greatly perturbs him:
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown,
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding.
Although Macbeth gains power when he takes the throne, in a way, he loses more. He loses his peace, compassion, and all sense of right and wrong. He becomes tormented by his insecurities and his belief that others will attempt to dethrone him.
Macbeth takes the witches' prophecy too seriously and decides to kill Banquo and his son. This will lead to many more misdeeds on Macbeth's part because he has forever lost his common sense. He wants to get rid of anyone who could potentially harm him in any way. This will lead to his imminent downfall.