What a long, sad trip it's been

I may have gotten this wrong since the ep wasn't really holding my attention, but I seem to recall that the demon said that Sam had never even known the real Brady at all. The demon had possessed Brady for the specific purpose of worming his way into Sam's life, so whatever memories Sam had of Brady as an "old friend" were all based on deception. That isn't to say that it wouldn't be merciful to free an innocent person from possession, but it wasn't mercy I saw on Sam's face there at the end, it was anger and revenge.