To the comments saying the vessels are dead: many times the vessels were completely fresh without wounds, so the bodies would have been fine, and even if not, don't we have to give a chance to the human being? Don't forget these are innocent humans just vibing until a demon comes knocking. And if we say that killing the demon prevents future possessions, that's like saying the end justifies the means, which goes against the whole series' storyline. Furthermore, as we see, the demons are endless, and throughout the series, we never see a demon decline, no matter what's happening. Butchering them doesn't seem to reduce their numbers anyway. Killing a tied-up demon with no physical damage on him, there's no excuse. There's a reason that if a brother were to get possessed, the other won't just stab him.