Cain is Belial.
Proof? The meteor (Tyrael/Imperius/Whoever) comes hurtling out of the sky RIGHT AT HIM! Seems a little obvious now then, in the pretense that Archangels would never hurt the real Cain. Whoever the angel is....it is quite clear what they want. The Black Soulstone.
Also - In the black soulstone cinematic, Azmodan is speaking to Cain/Belial.
Azmodan - "You thought you were so clever, that you had outwitted us all. One by one, our brethren fell into your trap. But not me. I defy you. I know the Black Soulstone is the key, and it shall be mine. Soon my armies shall pour forth from the Shattered Mountain ravaging this world and all hope of resistance. My minions will find the stone wherever you choose to hide it. Then at long last, Azmodan shall reign as the Prime Evil."
Go back and watch it again. You'll soon realize that I'm right.