Originally Posted by Nyloth

No, he first says that he can become a coven leader, and then he says, "well, or we can just kill him." And he wants to control the worm, but why? He likes the power.
It was my understanding that Astarion wants to control the worm, because it's the only thing preventing him from falling under his master's compulsion (don't D&D vampires have some mind controlling powers?).
Which could explain why Daisy appears as Cazador in his dreams; it's to scare him into using the worm powers. Later dreams she might tempt him by offering to make him more powerful than Cazador instead.