Darkness can be alluring to many, but most have the will and the courage to restrain. Some, however, succumb to its might. Here is a list of some heroes from Marvel and DC who at some or the other side gave into darkness:
Hal Jordan Becomes Parallax
This happens after Mongul destroys his home town, Coast City. This event leads to a whole new Green Lantern, or as he calls himself Parallax. He gives into the dark side and does a lot of bad stuff including killing his former friend, even Kilowog.
This might be somewhat of an argument again, but in the Dark Phoenix saga, when he tries to save his fellow mutants, Cyclops himself gets taken over by the Phoenix force, and under its influence kills Professor X, and gets into numerous fights with his earlier teammates. Not only that, he attacks humans and mutants alike with his new team.
Ant-Man (Hank Pym) has had a mental history. This can clearly be seen at the time the dude is mad at the Avengers for kicking him out. After which he even sent a robot to attack his former teammates. He also gave himself a name, Yellow jacket.
From Robin To Red Hood
Jason Todd, after coming out of the Lazarus Pit after the Joker apparently killed him was a totally different person. He was no longer the same sidekick Boy Wonder, he’s new came and made a name for himself as the Red Hood. H used to handle the city’s crime in his own way, which also involved beheading criminals.
Jean Grey after saving her team and dying comes back as the Phoenix. From being the weakest member of the team she comes back as an entirely new and strong person, however, her powers are not in her control, even so, that the Hellfire club tried to manipulate her into becoming one with their team, but they failed and eventually, she became the Dark Phoenix. She was so strong that she destroyed an entire solar system. After which she killed herself to save the worlds from further devastation.
Captain America Is HYDRA
This has to be the greatest shock to all the Marvel fans out there, telling people that Cap is and has always been a HYDRA agent, not just that but also a Nazi. Fans, as well as the people in the Marvel universe, are all enraged by the decision of this editors.
Superman The Dictator
The boy scout nature of Superman is nothing new to the fans, but in Injustice: Gods Among Us, the Man Of Steel kills. He, when under the influence of a drug given to him by Joker, kills Lois Lane and his unborn child, that lead to the destruction of Metropolis, snaps. And that’s bad, he starts by killing Joker and then creates world peace by enforcing it and eventually has to be stopped by Batman who brings a Superman from an alternate universe.