The Three Witches
Until Macbeth meets the witches, he's our hero. Once he meets them, he's already on his way with "imaginings" of murder. Agents of darkness themselves, the witches can be seen as guides leading Macbeth down the path of evil.
When Macbeth does some ruminating on killing Duncan and decides, "We'll go no further in this business," his loving wife turns him around pretty quickly. Macbeth appears to borrow Lady Macbeth's "undaunted mettle" to get the courage to go through with the murder of Duncan.
But, like any mentor or guide, Lady Macbeth can only go with him so far before she stops being useful. As Macbeth gains experience, he stops consulting her. The murder of Macduff's kids? That's on all him.