i'm hoping castiel is lying, i'm hoping i'm wrong and it's not a cain and able thing.
I don't see where sam's powers are so evil, and dean's kind of a dick as proven in yellow fever, if not 100 times previously, (a v. v. loveable dick) and i do think at the very end it will be sam and dean against the world, ( i also believe dean when he says he thinks the world's going to end bloody).
but for now... dean saying "GOD doesn't want you to do this" makes me more uncomfortable then him choosing flashlight duty.