All fair points, though more could be said for player contribution to such scenarios; it's not always up to the DM whether death is on the line. Everyone, and I mean everyone , knows: "don't bet against a Sicilian", et al. Beyond that, encounters where the best course of action is not metagaming is often the most satisfactory option when creating scenes for knowledgeable players. I agree, though, that a TPK should always be a surprise to everyone at the table and a mutual decision to not pull the ejection ripcord on the deus ex/mcguffin.