Beware of unmarked spoilers
Professor X is the reason the world is so terrible.
He used his powers to make little things go wrong around the world, ranging from Magneto losing control of his powers, to the Silver Surfer killing himself, most of the reasons why the world is so fucked is because he made it that way. The Scarlet Witch is the reason the world is so terrible.
Think about it. The Scarlet Witch is the only character who escaped unscathed for "turning in state evidence." And what is Scarlet Witch's power? Making unlikely events occur. Captain America of this reality was William Burnside
The original Steve Rogers might have been cannibalistic in this reality, but let's face it. Way too many things went right for Captain America to become a part of the Avengers in the mainstream continuity, which seems unlikely for a reality where everything is supposed to go wrong. William Burnside was still Captain America's replacement throughout the 1950s and 1960s and it was his bulletproof shield that's in the Quinjet that explodes at the beginning. The original Steve Rogers either died in the explosion that took out Bucky or he's still frozen. Nick Fury said that he "hasn't seen America in a long time." This could be hyperbole or maybe he meant the real Captain America. And William Burnside's psychotic nature would perfectly fit the grim nature of the setting. The fates of Marvel characters not addressed in this story.
- After he ended up on Earth, Howard the Duck was murdered on sight by startled humans who assumed he was a hostile alien invader.
- Steve Harmon was killed during his first adventure in Dimension X/Ecch because of Toon Physics gone horrifically wrong.
- Jessica Drew died of radiation poisoning before her father could cure her with the serum based on irradiated spider's blood. Either that, or the spider serum cure only worsened her condition.
- Squirrel Girl died from rabies.
- Alternatively she never existed, due to how inherently fucked up this universe is.
- The High Evolutionary is a Doctor Moreau-like mad geneticist who never perfected his craft and can only make hideous animalistic combinations of humans and animals in his blood-stained laboratory atop Mount Wundagore using crude surgery and injections. His favorite creation he calls "Adam," a Frankenstein-like humanoid constantly rambling that he lost his 'special stone', and that he doesn't have a soul without it. Adam is also obsessed with finding the perfect mate and he abducts and then murders local women.
- Kang the Conqueror is a madman locked up at Ravencroft Institute. He claims to be from the future and came back to stop the events leading to the ruining of the world as well as several famous figures and rulers throughout history. Currently he pretends to be a Caligula figure; decadent, pompous, demanding royal honors and worship from all who listen to his ramblings. He spends his days tinkering with alarm clocks and gadgets, says he's trying to recreate the time machine so that he can return. He also paints his face purple, and wears fetish leather boots.
- Thanos on the other hand is perfectly like himself. The only differences is that he was too successful with his nuking of Titan. Everyone died in the blasts, including Starfox and Mentor. He never found the Cosmic Cube due to Rick Jones never having the knowledge implanted in his head, so he's having to resort to halving the population of the universe one world at a time, much like his MCU counterpart, except he doesn't stop at just half. He just kills everyone to please Death. He hasn't touched Earth because that place is practically dead already and would be a waste of his time and resources to cull it. He's also a necrophiliac in this reality and brings corpses to bed with him.
- Mephisto is also like himself, just presented much darker, more like popular depictions of Satan. He is the only being in this reality that is truly happy and successful. He's quite pleased at how all his plans and hard work paid off into turning this reality into the Hell it now is. He's eagerly awaiting to see what his "son" and messiah will accomplish.
- Wade Wilson ended up succumbing to his cancer and dying without ever going to Weapon X.
- Alternatively, he did go to Weapon X, but the healing factor he was granted dramatically worsened his cancer and he ended up dying from it.
- Peter Quill died in a freak accident trying to get to space.
- Rocket Raccoon is an ordinary rabid raccoon.
- Eric Brooks's mother was fully drained of her blood by Deacon Frost and she never even gave birth to Blade.
- Miles Morales and Kamala Khan were never even born.
- Perhaps one or both of the parents of each one were killed before Miles and Kamala could be born.
- Rom is the last remaining Space Knight after the rest of Galador's infantry was wiped out and has been driven insane by his 200-year war with the Dire Wraiths, the trauma of failing to save innocents or his homeworld from the Dire Wraiths making him resort to murdering his foes instead of using his neutralizer to banish them to Limbo. He ended up obliterating all of Clairton due to believing everyone there to be a disguised Wraith, either not bothering to use his analyzer to confirm anyone to be innocent humans or assuming that his analyzer confirming the inhabitants' humanity was some kind of Wraith trick.
- Baron Karza thwarted the Micronauts' efforts to lead a resistance towards his tyranny and had them added as stock for his body banks.
- Instead of becoming the supervillain Electro, Maxwell Dillon's accident resulted in him being electrocuted to death.
- Flint Marko was atomized by the nuclear detonation that turned him into the Sandman in the regular Marvel universe, his only remains being inert sand grains.
- Curt Connors died from complications to his body caused by his lizard mutation side effect from regrowing his amputated arm.
- The Power Pack did not survive their first battle with the Zn'rx. The Zn'rx simply ate the children, and left, seeing Earth as too far gone to conquer. The Power Pack’s parents, unable to cope with the deaths of all four of their children, were Driven to Suicide.
- Considering how few adults became heroes (Reed and Sue Richards never did), and how poorly even adults can control their powers, most of the pack, especially Franklin and Maria were probably never born, and the group never formed because the other members died as a result of their powers or were locked up.
- Eddie Brock was consumed by a symbiotic parasite, dying from it eating his brain.
- As the last page reveals, Peter Parker never became Spider-Man in Ruins, he succumbed to the bite which gave him a contiguous disease instead. There was also almost no possibility of any Secret Wars, so no symbiote could ever have been brought to Earth. Therefore Eddie Brock would have never encountered any symbiote of any kind, let alone have it eat his brain.
- Chris Powell was gunned down by criminals before he could become Darkhawk.
- The Runaways found out their parents were supervillains and were killed by them for finding out.
Ruins takes place in the Dark Multiverse.
The Dark Multiverse is a reality born out of the nightmares and fears of the regular multiverse, where everything eventually goes horribly wrong. Ruins is a what if where every origin story ends up going horribly wrong. Ruins is a universe born from the collective fear of every hero and villain who was transformed and dislikes their powers and status ie the Thing not liking being a rock monster or Bruce Banner and the Hulk's difficult relationship. A Hell-Lord is responsible for this world's state.
Through some unknown means, a demon lord (probably Mephisto) managed to alter this universe's timeline to turn it into something more hellish, where horror instead of marvel is this world's reality. Worse still, it may have been the One Below All itself.- As various past comic appearances have shown, Mephisto is already largely responsible for many of the famous disasters and wars throughout human history in the Marvel universe as human evil and suffering empower him. He would just need to be slightly more subtle and successful in his usual methodology, and he could easily bring about Ruins.
Nyarlathotep is responsible for this world's state.
Alternatively, the cruel trickster of the Cthulhu Mythos somehow discovered the Marvel Multiverse, and decided to twist and corrupt one of its iterations to see if it was possible, making it much more like his own Cosmic Horror Story cosmos, filled with body horror, insanity and hopelessness. He may have done it simply for his own twisted amusement, or for much more terrible and insidious reasons... This world was created as a stress relief for the Multiverse.
Every time a superhero gets powers that break the laws of physics or time travel gets too convoluted, it adds stress to the Multiverse. Ruins and other dark universes are created by The One Above All to bleed it off and let the multiverse continue as is.