for Macbeth's visit. She says that "everyone shall share i' th' gains" (4.1.40). It is strange to imagine what the sisters have to gain from their interactions with Macbeth, and I cannot think of anything other than amusement at his own suffering and the suffering he causes others. This interaction, at the least, proves that the sisters and Hecate have prepared some kind of plan for Macbeth, and it certainly doesn't seem positive.

In the rest of the scene, the Weird Sisters seem to intentionally offer Macbeth enigmatically-worded statements of destiny rather than the assurances of security that he believes them to be. He believes it is impossible for a man who is not born of woman to exist and impossible that a forest could move; their statements make him feel invulnerable, which he is not.

A good place to look for lines which support Macbeth's being a victim of the three witches is Act 3 Scene 5.  In this scene, Hecate scolds the witches for telling Macbeth his fate.  Hecate calls Macbeth a "wayward son" which suggests that he is too easily swayed by his own ambition and greed to make the right decisions regarding his future.  Hecate tells the witches that they have been "saucy and overbold" in their dealings with Macbeth and that they must correct the situation.  They all know that Macbeth plans to visit the witches for another foretelling, and Hecate commands the witches to cast a spell.  Since Hecate is scolding the witches for their dealings with Macbeth, the reader assumes that Macbeth is their victim.