Sometimes for character development, while sometimes to being something new, the writers bring about different changes in the arcs of our beloved heroes. Sometimes even making them the very villains that they are supposed to fight. Here are a few examples of such cases where superheroes turned villains:
After bonding with the symbiote Spidey turns a bit dark, growing violent and a bit out of control. He even fights crime in his sleep at that time. At that time he was even feared and hated by the citizens that he protected. The symbiote eventually detaches from him and sticks to Eddie Brock, creating Venom.
Jason Todd after being killed by the Joker, seemingly dies. Only to return as the Red Hood years later. The anti-hero uses guns and killing nd is severely angry at Batman for letting the Joker live even after all that he had done. Todd then is briefly put inside Arkham, and now his kill spree is somewhat stopped.
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.
Originally Captain America’s friend and Sidekick, Bucky Barnes after being thought to have died during combat reappears as the Winter Soldier, a reprogrammed Soviet assassin. Bucky now had no memory and committed a lot of political assassinations at the time of the Cold War.
This is the most surprising of all, where Superman in the Injustice comics is pushed so much that he becomes a narcissist. In the comics Oleson is killed, the Joker manipulates Superman into killing Lois Lane who is impregnated with Superman’s child. After which Superman punches a hole through Joker and Batman has to bring their counterparts from an alternate universe to stop him.