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?).
Exactly. He is interested in powerful objects and the tadpole because he knows only something powerful can truly free him from Cazador.
I mean, he is quite low in Maslov's hierarchy of needs right now.
I can't find the dialogue right now, but I think I remember that he says he could become a full vampire, but it's enough for him to just kill Cazador. That's why I think he just wants to be free at any cost and becoming a full vampire is simply one of ways to achieve this goal.