In this scene, Macbeth demands information from the witches about his future. His goal is to use this information to protect his power, not to make a logical, let alone an ethical, decision. Given the first two visions, Macbeth determines to kill Macduff and his entire family to eliminate Macduff as a threat.
With the third premonition, the apparition (in the form of a child crowned with a tree in his hand) tells Macbeth:
Macbeth shall never vanquished be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (IV.i.108-10)
Macbeth replies that this "will never be." He ignores the image of the child holding the tree which is a clue to how the army will disguise itself with branches, thus becoming a moving forest. Macbeth is too focused on his own fortunes to notice this. Macbeth is unsettled with the fourth image of Banquo (his descendants) becoming king because he expects the witches only to give him prophecies that serve his greedy interests.
In this scene, we see Macbeth completely single-minded in seeking to sustain his selfish desires. Even his guilt and anxiety, which is evident in other scenes such as Act 3, Scene 4, is absent in this scene. At this point, Macbeth has become even more determined to sustain his power and status as king. Once the witches have vanished, Macbeth tells Lennox that those who trust the witches should be damned, essentially damning himself. This is a key moment because although the witches provide Macbeth with warnings and potential futures, it is Macbeth who chooses to act on them. In doing so, Macbeth is ultimately responsible for his sins.