However, as entertainment reflects society, is the true culprit society and politics, itself?

Everything from comics and TV shows to wrestling and cartoons appeared to take a more provocative and darker turn as we entered the 1990s. That alone seems odd to me because in 1989/1990, the Cold War ended, George Bush talked about a "new world order* and the Berlin War fell. Surely the 90s should have been a more positive decade if (and it's a big if) if entertainment is supposed to reflect reality.

*No, I don't believe in the conspiracy of the 'new world order', I am merely using a phrase many have described when talking about peace/prosperity.

>> Is there a first domino? <<

I'm going to suggest that maybe it was Neal Adams.

The realistic style he unleashed in comicbooks changed virtually everything, influenced virtually everybody, and not just artists, but led by the artists. A writer could then easily begin to think, well, if my artist can come up with these kinds of images, supporting these kinds of plots, well, I can write so much that's never been seen before.

I think the dark really started with Frank Miller's DAREDEVIL.  Daredevil holding a gun to an immobile Bullseye's head on his hospital bed--that's dark.  Matt Murdock acting weird and driving Heather to suicide--that's dark.  I loved Miller's work on the book, but he did take it too far a couple of places.

Jean was not even an official member of the team during the Dark Phoenix saga, and Speedy was a supporting character that had disappeared for a while.  It's different when a major talent turns the STAR of a mainstream super-hero comic dark.

Denny O'Neill and Neal Adams "relevance" stories were certainly a tipping point as they confronted problems without any resolution, something of a first for super-hero comics. Being unable to restore Kandor or permanently keep the world safe from Doom is not quite the same thing as facing the population explosion, pollution, and drug abuse. 

Miller's Daredevil run gave the comics world something it never knew it wanted until it was suddenly there; Noir. Borrowing heavily from Eisner, Miller employed storytelling techniques never before seen in the Marvel Universe and upped the level of brutality considerably. DD also gave us the Kingpin, who did evil all the time and could not be stopped from doing evil. Doom, the previous "Nyah nyah, can't catch me!" villain, did good in Latervia, or one could argue he did anyway. The Kingpin ran an empire based on terror, murder, and drugs and was fine. Absolutely fine. No one could touch him... That was something new.

This led to Amazing Spider-Man Annual #15 in which the Punisher is the bad guy, or is he? He certainly seems cooler than everyone else in the book and his personal moral code plays perfectly into the story. He could kill the cop and get away. But that would be killing a cop, and a young one at that. So he goes to jail instead...

Where he gets his own mini-series. Wolverine got his own mini-series as well. Suddenly heroes-who-kill seem like the coolest thing ever. Killing people used to be wrong... Now, not so much. You just have to know who to kill and everything's ducky.

Then came Dark Knight, and it wasn't second-tier characters and team players who were the killers. It was one of the big boys, and he was more violent than anyone had ever imagined him being. Fandom went agog. Super-heroes could be seriously bad-ass. Cue more darkness for Batman, mainstream, elseworlds, whatever... Just turn out the lights and bring it on! The Killing Joke; The Cult: Arkham Asylum... Hey, what if Batman wins all the time because the others aren't half as crazy as he really is? That would be bad-ass, right? Fuckin' A...

Soon, every costumed clown who ever went up against a super-hero was a certified and certifiable serial killer, with DC setting up an entire summer event, Underworld Unleashed, to turn nobodies into slash-happy, giggling psychos. 

Reboots helped tremendously because no one was answerable to the past anymore. "Heroes didn't used to be like this!" wasn't a valid argument anymore. These had. Superman has always lived in a world of mass-murdering nutjobs and Bat-Ninja has always been one step away from being the worst of them. Always always always. Cue Red Diana, She-Amazon with a Sword, who was restarted as a pacifist, but, y'know, the kind of pacifist who chopped the heads off her foes, Cap vs. Baron Blood style. Soon, pacifism had nothing whatsoever to do with who she was and why she was here. Screw that. Choppemupanfeedemtothedogs!! There's blood to be spilled!! Can I get a "yee-haw" out from comicdom? Yeeee-Hawww!!

Exactly which domino did the most "damage" is dependent upon what one sees as the actual problem. GL/GA were good men in over their heads, facing problems we all face and trying deal with them on some level, somehow. Daredevil was a good man in a world almost entirely absent of hope. DD's New York was a bad city and getting worse, and he was just some impotent schmuck, who when the time came, had no bullets in his gun. He's still not the bad guy. Wolverine is. The Punisher definitely is. They kill and feel no compunction about doing so the next time they think they need to. Also, no consequences for them! Kill, kill, and kill again, and next morning, what's for breakfast, Kitty? Bad guys. Nonetheless, they set the stage for others to follow. Ghost Rider for one. "Penance Stare?" Ye-ahh, right... How about just a grisly death instead? And one for you, and one for you, and one for you...

Along the way we had Moore's Swamp Thing and Vertigo and comics written explicitly for adults where anything goes, the more extreme the better. Cue the welcome reception of those then funneling those creators and their sensibilities back into the mainstream heroes and a world where Batman and 100 Bullets might as well be one and the same book... 

Yes, once upon a time, there were Golden Age heroes who killed, but their's was a different world, one without all of this "continuity" and desperate need to follow implications to their logical ends. If the Comet killed, it meant the world in which he lived was a brutal and ugly one. When he died, his brother took up the calling and hanged the bad guys by the neck. Right there, in the same comic book however, you could read about busy, industrious nurses or daring pilots and funny animals. The Comet living in an ugly world did not mean that everyone did. 

Not so today. Hell, untold googleplexes of innocent lives were taken just so we could all link our comic books together and say, "This one links up with this one, and this one links up with that one..." Now there is no character untouched by the unpleasant tone and musty, sweaty odor permeating the Marvel and DC Universes today. If you want to do a story with a talking ape, that talking ape has to know this talking ape and the other talking ape... And of course, the one that kills people indiscriminately... So, in a hilarious "twist," this month, we'll break Dumb Bunny of the Inferior Five's neck for her, on-panel! Ho-ho-ho!! Funny, right? And Detective Chimp, who once upon a less-continuity-conscious time lived in world with a tone approximately that of an innocuous Disney live-action flick, now lives in this world... And it sucks. And everything sucks. And so he drinks. Because, what the hell else can you do when you're a talking monkey living in a world where Dr. Light raped Sue Dibney and then later her best friend killed her and set her on fire? Once upon a time, Ralph and Sue lived in a world like that of a light-hearted detective film. Now... They live in this one. Or rather, they don't, and everything sucks all the more because of it... 

People like to say, "I didn't make the world; I just live in it," but every time one of these comics come out, yes, the creators help make this world, the one we all have to live in, a darker and less appealing place. They take symbols of what could be a brighter future and cake them in filth and encrusted blood, because, hell, they look good all dirtied up like that! Buncha' infantile concepts parading about like they own the place, like they're something better than the rest of us... Well, they don't own the place, now do they? And when we, the creators and the fans, tell them to jump up and slash someone in the throat, they damn well have to do it! Now, there's a power fantasy for you, boys and girls. And you're welcome.

The Old Marvel Guard put on the breaks within a few years (as JB has noted many times), but it seems perhaps that the damage was done in ways nobody in the mid-60s could have reasonably foreseen.

Was Stan Lee (whom I worship) the one who set things in motion? By writing the kind of of stories that appealed to himself as a reader, was the eventual "adultification" of the superhero genre inevitable?