Some of these stories are well over 20 years old, but just to be safe: there be spoilers here.
Rick Remender started Uncanny X-Force with an arc where the secret strike team of mutants killed a child version of Apocalypse. Little did they know, Angel was the heir to big, blue, and ugly, which led to The Dark Angel Saga. Angel transformed into a rather badass version of Archangel and rallied his own Four Horsemen in an effort to wipe out the world and rebuild it in his twisted image. He almost succeeded, too. This embodiment of Apocalypse was determined and put forth his plan with deadly accuracy, but he forgot about a little thing called love.
Angel's lover Psylocke went so far as to become his new Death, and almost succumbed to evil herself, but in the end she was able to break free and defeat him with the mysteriously powerful Life Seed. The taint of Apocalypse was eradicated, but it left Angel without any memory. The entire Dark Angel Saga was enthralling, exhilarating, and extremely violent from beginning to end, and that is what has made it an instant classic. In fact, we recently gave it the distinction of being the Best Story Arc of 2011 in our end-of-the-year awards. Uncanny X-Force continues on, and we cannot wait to see the consequences of Angel's actions and how Psylocke will deal with erasing the man she loved.
Terra was a great character who provided a lot of friction to the team. Once readers found out she was evil, every issue leading up to her eventual demise had a juicy layer of tension on top. Beast Boy fell for her but Raven hardly trusted her in the first place. Even though Raven was right, it was Beast Boy who took it the worst because he truly believed her to be a hero up until her final moments.
Marvel revealed a lost Stan Lee Silver Age super hero. It was a hoax, but it was also a theme for the Sentry that what you see is not exactly what you get. The Sentry was said to have the power of "one million exploding suns," and while we are still scratching our heads to what that means, he was a likeable character named Bob who slowly discovered he not only had super powers, but he once had ties to Marvel's most famous heroes. However, as he began to recover his memories, a dark force called the Void was also regaining its powers.
In a one-shot called The Sentry vs. The Void, it was revealed that the Sentry and the Void were two parts of the same person. It turned out that Reed Richards and Doctor Strange made Bob and the entire Marvel Universe forget he was the Sentry in order to rid the world of the Void. At the climax of the issue, Bob once again sacrificed the memory of his life as a super hero to defeat the Void. It might have started as a hoax, but it ended up being an incredibly well-done thriller that also paid homage to Marvel's Silver Age.
Hal Jordan's fall is one of the ugliest on this list. Readers felt for him as he tried to deal with the murder of the seven million people in his hometown, but in his rage he ended up becoming a murderer himself. While he would eventually go on to exonerate himself due to the influence of the fear entity Parallax, the Emerald Twilight story arc remains one of the most intense, brutal, and nasty stories in Green Lantern history.
Ed Brubaker transformed Bucky from a joke into something cool and dangerous, which made his return after his apparent death all the more nefarious.
The Winter Soldier - an elite Russian assassin with a domino mask and a robotic arm - was committing countless assassinations, and it was up to Cap to stop him. Cap came to realize it was Bucky, but he could not talk any sense into him because of the extensive brainwashing by the Soviets. It took the power of the Cosmic Cube to bring back Bucky's old memories, which caused him to feel tremendous guilt for the atrocities he had committed.
Eventually Bucky came to terms with what he had done, even going on to don the mantle of Captain America himself. But his story of manipulation, identity, and second chances has defined him as a forever-troubled character with a violent past.
DC Comics publishes stories about superheroes, and he just happens to be named Clark Kent and discovers he has Superman's powers. Through a lot of elaborate plot devices, he helped save the universe, and then he wound up in a pocket dimension. From there, he secretly watched the world's heroes lose their way and, in his mind, become aberrations of the heroes he grew up reading about in comics. It drove him insane.
And so he punched the wall of reality during Infinite Crisis. Yup, the infamous punch that brought Jason Todd back to life and changed several origins. After a mind-bending journey through time and space – where he destroyed existences and battled himself – he somehow wound up back at his home. Unfortunately for him, everyone had read about his villainous exploits in the comics and knew how evil he was, giving him yet another thing to whine about.
As part of the Pride's ritual for the Gibborim, only six people were allowed into a new utopian world, and Alex planned for those spots to go to his parents along with his love interest Nico and her parents. Too bad for him, Nico's response to his plan was a punch to the face. She helped rescue her friends and fought against Alex, which resulted in the magical Gibborim killing both Alex and the Pride.
Shadowland storyline. He constructed the Shadowland temple from the ruins of a building in Hell's Kitchen and dolled out justice how he saw fit while wearing his new black duds. His most shocking action came when he murdered his greatest rival Bullseye the same way Bullseye had once killed Elektra.
Murder doesn't really fly within the superhero community, so Daredevil was besieged by the likes of Spider-Man, Ghost Rider, and Iron First, who discovered that he was being controlled by the demonic Beast of the Hand. After his best friend Foggy failed to get through to Matt and most of the heroes had fallen in battle, Iron Fist managed to deliver a healing chi blast to the possessed Daredevil.
This gave Elektra a chance to help purge the demon from Daredevil and whisk him away to safety. While the event received mixed critical reception (landing on IGN's Worst Comic Book Events list), it was a landmark event for the character that showed just how dangerous Daredevil could be.
Mark Waid. But Waid is also responsible for one of the most heinous superhero stories ever told: Irredeemable. It stars the Plutonian, the world's greatest and most powerful hero. However, after becoming fed up with his personal relationship and the harsh criticism he faced from society, he snapped. However, when a Superman-class being snaps, it's not quite the same as a regular Joe throwing a tantrum. He killed his entire hometown of Sky City with a population of over three million people and then went on to sink Singapore into the ocean. Irredeemable? Sounds like the perfect title to us.
Waid has built a whole series that examines the concept of the fallen hero. Plutonian had the best powers, a girl, and a world of respect, but not the mental capacity to handle the hardships that come with being a hero. If you thought sinking Singapore was bad, just wait until you see what he does to a group of villains who try and recruit him, or a young super hero who attempts to reason with him. One thing is for sure… it ain't pretty.
She was just having a snack, but at the cost of billions of lives that inhabited that star system. This caught the attention of the Shi'ar Empire, who wanted to execute her for her murderous actions. In X-Men #137, the X-Men fought the Shi'ar Imperial Guard in a trial-by-combat, and when Cyclops took a hit, the Dark Phoenix emerged once more. In a desperate act of heroism, Jean Grey took control long enough to kill herself with a Kree disintegration weapon.
Uatu the Watcher closed out the story with the now famous words: "Jean Grey could have lived to become a god. But it was more important to her that she die... a human."