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?).
We don't know for sure. We can only guess at his true causes. But when you talk to him, he first says that he can drink the master's blood, and only then adds that we can also just kill him. Doesn't that indicate his priorities? By the way, I strongly regret that I looked at the spoiler... I mean, you're talking about something that's not officially available.
Could you please ... not answer me with that information?.. Thanks.