Sam only exorcised demons until Alistair, who he actually killed. He killed Lilith, too, at a minimum.
With the knife and the Colt, when a demon is stabbed, they kind of light up from within. The Colt goes a step further and does that skull flashy thing and YED's eyes even formed skulls, I think? The flashing with the Colt is also brighter and paler than the flashing with the knife, which is more muddled and reddish.
It kills demons, yes, but we're not talking about them going back to Hell. We're talking about if they could end up in Purgatory (since they aren't human and that's where non-humans go when they die), which I see as possible with Sam's powers and with the knife - though maybe not with the Colt. :)
Re: John's and Mary's absence from heaven (small spoilers)
Sam only exorcised demons until Alistair, who he actually killed. He killed Lilith, too, at a minimum.
With the knife and the Colt, when a demon is stabbed, they kind of light up from within. The Colt goes a step further and does that skull flashy thing and YED's eyes even formed skulls, I think? The flashing with the Colt is also brighter and paler than the flashing with the knife, which is more muddled and reddish.
It kills demons, yes, but we're not talking about them going back to Hell. We're talking about if they could end up in Purgatory (since they aren't human and that's where non-humans go when they die), which I see as possible with Sam's powers and with the knife - though maybe not with the Colt. :)