When we were kids playing D&D, we were just doing what the modules told us to do .
Not always. The bad DMing habits and behaviors Gus identified were sometimes encouraged by some of the advice in the modules or Gary's DMG, but most of the things he listed are simply errors by DMs. Sometimes published adventures included such errors, but they still would have been recognized as bad DMing at plenty of tables back in the day. Because they simply make the game un- or less fun.
Also there's a lot of Presentism clouding how we view older games. Look at the popular games - 5e and Shadowdark. They're both super-protective of the PCs. "Death saves"? Really? It's completely unfair to judge old-school DMs by the modern standard because games weren't as soft then as they are now.
Eh. AD&D 1E is softer than 0E or B/X. PCs start with more HP, they get higher ability scores and more bonus HP from Con, more healing spells (starting at 1st level, Cleric PCs usually have 2-3 first level spell slots, compared to not getting one until 2nd level in OD&D and B/X), and they get the rule where you don't die if knocked to exactly 0HP (optionally as low as -3), but instead begin bleeding out at 1HP per round until you hit -10.
In practice, a ton of tables ignored the 0 to -3 range and just played it simply that anything short of -10 left you bleeding, which is how it was described as a optional rule in 2E.
While it's true that the older TSR rules made it easier for PCs to get killed accidentally in combat, it's also true that the trend of PCs getting tougher and
recovering faster was a continual trend starting from the first rules revisions in 1975.
Trivia: The first Death Save rule for being at 0HP (as opposed to Saving Throws vs Death, which was originally for Death Rays and death spells and quickly expanded to other instant death effects like poison) appeared in the 1991 D&D Rules Cyclopedia.
I'm also defending the DM using DMPCs because usually it was someone who wanted to play but couldn't because no one else would DM. The DM chair's a lonely place when you really didn't want to be there in the first place. IMO if a player takes one for the team and becomes DM, we shouldn't begrudge them their overpowered and obnoxious pet PCs.
We can cut them (and our adolescent selves) some slack about the reasons while still recognizing that obnoxious and overpowered are descriptors we don't want to apply to stuff in our games.
Finally, how hard is it for a GM to kill off a party anyway? Super-duper easy. They could do it before combat even starts. GMs don't kill PCs. PCs get perished because (1) the player made bad decisions, and (2) the dice rolled against them. And it's funny because when GMs interfere with player agency, it's an unforgivable offense, but when PCs die from players making bad decisions, it's STILL the GM's fault.
I think you're conflating different things. Sore losers blaming the DM for their own mistakes are certainly a thing, especially when we're talking adolescents. But "killer DMs" making the game unnecessarily hard and unfair are absolutely also a thing. Also especially when we're talking adolescents. Gus enumerated several real ways DMs can accidentally earn a negative reputation through bad refereeing. I've seen all of those forms of bad DMing in real life. Though thankfully most of the DMs I've played with learned and grew over time (as did I, especially in DMing skills).