Aspiring Lurker (He/Him)
As an aside, I wish we could get away from the idea that the GM kills the PCs. That’s not fair to put that responsibilty on one person. The GM presents interesting situations/problems/obstacles and describes how the world reacts to what the PCs do. If you do stuff that’s likely to get you killed you are likely going to get killed sooner or later.
I agree with @the Jester that in this case the characters seem to be committing elaborate suicide. While I think it's possible for a GM to go past any semblance of fairness in the direction of killing the characters, I don't think that's happening here.