IME there are no "killer DMs" - just "meat-grinder" adventures/campaigns.
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While I agree that adventure content has a good deal to do with how deadly a given campaign is ... there are absolutely bad refereeing methods that make for "killer DMing". many of us may remember some of these from playing D&D when we were pre teens and teenagers in the 1980's...
1) Having exactly one right solution to a deadly problem. This is more then just deciding on a possible solution to a trap or similar encounter, it's the habit of then rejecting any other ideas out of hand and killing the PC who attempts them for doing it wrong. Players think of things the referee and designer doesn't all the time and often these also work to solve puzzles or neutralize dangers.
2) Playing "Gotcha" and "Mother May I". This problem has been reconceptualized a bit lately in the contemporary traditional space to encompass almost any sort of danger that isn't entirely obvious. What it means here is hiding obvious dangers through omission. E.g. "HA HA! You fall into the lava, roll 12d12 for damage ... you didn't ask me what the floor looked like." This is why referee clarification is important, because otherwise the players have to clarify EVERYTHING and that takes forever. Referee description is the players only way of knowing about the world and one should be generous rather then hiding things with omission.
3) A compete lack of telegraphing or clues. This isn't to say you can't have traps in hallways - put generally traps or monster ambushes that have no possible means of discovery (or require some sort of excessive caution - see 2 above) shouldn't be immediately deadly - at least they shouldn't be common. This doesn't apply to obviously dangerous situations such as treasure chests and the like (a poison needle is a chest is a classic), but even "hallway traps" that aren't a save vs. death, such as a pit or spear trap should have some kind of warning, even just the knowledge that the dungeon is trapped ... or that the corridor is entirely unused (dungeon denizens don't like skipping over their own tripwires). The same goes for monsters with dangerous special abilities. Poison drips from the jaws of a "poison wolf" (not necessarily from a spider though ... because spider) etc.
4) DMPCs ... not just the ones that help the party - those are annoying as hell on so many levels ... but I means enemies and such that the referee has a special fondness for. In the old G+ days one party playing Ravenloft famously got very lucky somehow blasting Strahd when he first appeared to mock them. This is not how the adventure is meant to go (and it's hard to do), but the referee should let this kinda thing happen when the dice say it happens. NPCs and foes - even the ones with fun plans have to be mortal and have to play by something akin to the same rules as the players (not exactly the same - they need not have the same abilities). They might take precautions (shield spells, magic items, demon pacts that make them immune to normal weapons etc) but if the players get lucky or figure out a monster or NPCs tricks and kill them, the referee should let it stand. A killer referee won't just let their pet NPC escape though, they will use the pet NPC to punish characters who try to beat them and especially when they do something clever. This is bad refereeing.
5) Antagonistic Refereeing more generally. D&D/RPGs aren't a wargame in the sense that they are meant to be more or less equal contests between player antagonists governed by rules of a game. The DM is not a player (one reason I like "referee" not any kind of "Master"), despite what Dragonlance's DL1 Dragon's of Flame says in the intro...(read it you will not thank me, but you may chuckle). The referee has the power to manipulate the setting and rules of the game almost any way they want, especially in older systems. This has can't be brought to bear against the players in an antagonistic way - the referee can't win. They can't lose either. Thinking of the game as a contest between players and referee is the best way to ruin things. It's tricky of course, as a referee one wants intelligent enemies to do intelligent things, to use their advantages - and that's fine - but as a referee one has to temper this with accepting that dumb enemies do dumb things and that even smart ones might make mistakes or otherwise fail to plan for every eventuality.
Anyway that's enough maundering from me. But yeah - killer DMing is a thing, and the more dangerous an adventure is the more it's something to avoid. I think it's especially an issue these days with people new to "old school" style stuff buying a bit to hard into the idea that the game is meant to be super lethal to PCs or especially unforgiving. To some extent this is even a hold over from newer editions where organized play tends to be designed with more wargame like scenarios and the balanced tactical combat focus with high character power level/survivability encourage referees towards a more antagonistic style out of a need to make things challenging. Older editions have no problem creating super challenging situations.