I think I see the Dark Cell as a sort of treasure keep or storehouse. Where rather than a testing place, the ordeal is apparently set up as impossible for anyone but the Master.
Whatever treasure is inside, the Master has access due to a keycard or somesuch. The rest of the poor yokels are typically forced off the property with a patented reverse vaccuum (Phase 1). If they don't scram from that, there comes the illusory razor wire (Phase 2). If they make it past that, the last test is impossible, basically a brick wall preventing passage (Phase 3). Beyond Phase 3, isn't a real phase, because you aren't actually supposed to pass here. Looked in this way, yes, it would definitely make sense to try to kill someone who actually made it somewhere they shouldn't, but before that, it would be overkill to kill people who discovered it by accident, and if was pretty much impossible to get in, no other security would normally be needed. Anyway, that's my take on this. We will have to see.