But there's nothing to say that Sam's powers or the knife causes a death of the soul
WRT to Sam's powers: I always thought he was just exorcising them. Because we always see the smoke coming out and sinking into the ground. When they're killed with the Colt or the knife, that doesn't happen.
WRT to the knife: I think that would also cause a soul death. It's always known for being the thing that KILLS demons. Sending them back to hell is never considered killing them.
Re: John's and Mary's absence from heaven (small spoilers)
But there's nothing to say that Sam's powers or the knife causes a death of the soul
WRT to Sam's powers: I always thought he was just exorcising them. Because we always see the smoke coming out and sinking into the ground. When they're killed with the Colt or the knife, that doesn't happen.
WRT to the knife: I think that would also cause a soul death. It's always known for being the thing that KILLS demons. Sending them back to hell is never considered killing them.