A demon will tell lies when convenient, for us, and truths when inconvenient, for us. There's no way to verify Kimblee's claims, that the demons will leave the bodies of our friends after this is over. Then what, indeed, is the point of all this?

However, we can't afford to lose Oswin and Bulat, and at this point, killing them is inconsequential to the demon, given his end has been determined. That is why I say that you're making the best decision here, my Chief.