I think part of the blame lies in the module system, and here's why:
- if a module is mainly about the puzzles and the monsters, and not about a story (least of all a time-sensitive one), then players are just there to be challenged and clever and explore lots of stuff. They die? Then I guess they aren't good enough, mwah haha! A challenge here for the DM is to think about what content is likely to be experienced and what content is likely to remain hidden or unknown - - you don't want to put too much work into an amazing set of traps if it's unlikely the players will choose that path in the first place.
[- - Being able to re-visit a given dungeon can reduce the "unseen content" factor, but of course in one-shots and the like, this just isn't a realistic goal]
- OTOH, if a module has a strong component of setpieces (things that are time sensitive and maybe sort of "mandatory" to get the "full experience"), then there's an incentive to make sure at least some of the players survive long enough to see them. And if we put our fancy hats on and try to design puzzles or "experiences" that require a certain minimum number of (living) players, or a particular class is required for some reason to get through it, then the DM is going to feel that much more of a squeeze to let everyone live, always.
[- - Setpiece-focused design is all about the performance. It's like being on a ride! Ultimately, the priority of showing off the module's content supersedes the priority of no-holds-barred exploration. When you start to figure out that the DM isn't going to let you die, it changes the dynamic, and not always for the good.]
You know which side of the fence I'm on! But that doesn't make it any easier - just because you're committed "ideologically" (so to speak) to a particular play ethos doesn't mean you can just snap your fingers and make it happen - apparently that takes practice.
Cases in point:
1- I didn't really think very tactically about how to play Ishigiri the Ogre Mage (homebrew: Dwarf Troubles Lv 1), and instead of him using his 1/day cone of cold on the healer, he used it on a henchman. The dice roll was pretty epic, nonetheless: I ruled that, due to extreme overkill, the henchman's feet broke off and remained frozen to the floor while the rest of his icy corpse went skidding along the tunnel, only to shatter against a door.
2- I did make sure that people had appropriate warning about the pit trap in the opening hallway (homebrew: Lair of the Cyclops Lv 1) - - "the floor's creaking", "the dwarf senses a trap", etc. Once you are given a warning, and you keep walking on the creaking tile floor, it's on your head ^___^ There's a trick to it - imo, in order to be a reasonably likable and trustworthy DM, you can be cruel, disgusting, and vicious, but only insofar as you have established reasonable credibility (a highly localized phenomenon) regarding your ability to be fair.
What is fair? I think it's a fairly simple matter of establish-then-execute. AW & DW talk a lot about this: do what the fiction demands. How do you know what it demands? Well, kid, the MC/DM moves tell you what you can put into the fiction and when, but the players do a lot of that for you and you just wait til they screw up a dice roll.
Creaking floor? OK, the pit trap is good to go.
Glowing eyes? The cultist's magic powers are good to go!
In the case of Ishigiri, I think a frost theme around his lair could have signaled that that's one of his signature powers. That, or fog-breath, or something. Hints like that are big, loud, alarm bells to the veteran adventurer, and something subtle really can establish your credibility enough (adventurers like to blame themselves for dying...)