Prue: All right, listen, here's the deal. Belthazor killed the Triad, now The Source wants him dead. Krell is trying to suck up to The Source, so he wants to kill Belthazor.
Piper: Belthazor wants to kill us so The Source won't kill him.
Leo: And if you work with Krell, you can vanquish Belthazor before he kills you.
Phoebe: Works for me.