Fallen

“	I did love being a hero… but if this is where it leads, I'm done with it.	„
	~ Justice Lords' Superman before he murdered Lex Luthor with his heat vision.

Many heroes walk a thin line between good and evil; indeed, many heroes have fallen into despair, rage or even madness, and once proud heroes descend further into the path of evil, often fighting against people they once protected or fought alongside.

Originally protagonists or forces of good, these villains turn evil due to events in their lives, such as being corrupted by another villain (i.e. convinced by villain's arguments), their desire for revenge, a failed love relationship, betrayal from the good side to the dark side, or any other unresolved traumatic experience in which they question whether or not the good side is worth fighting for. However, some heroes may turn for more greedy reasons (i.e. "From Sugar to Excrement" scenario).

A Fallen Hero - or hero turned to the dark side - is usually the end result of a protagonist exposed to a Corrupting Influence, Brainwasher, and occasionally an Extortionist, a Blackmailer, or even a Bond Destroyer. Some of them can still redeem themselves such as Niko Bellic or Injustice's Green Lantern.

There are various scenarios why these individuals turned to the dark side, though the said individual often to fell onto multiple scenarios altogether:

From Zero to Nightmare: a scenario where the said hero turned to the dark side in this way often became shocking events for either viewers, heroes, or other people that learned about this. Reason of this mainly because the said hero always looked down upon, his/her existence was ignored, and/or worse, considered as an outcast or an individual that often being abused by other people that dislikes/hate them (either the Big Bad or merely minor villains in the story) no matter what kind of good/heroic efforts that he/she did for others' acknowledgement. Once this individual turned to the dark side, they became feared by many people, includes heroes that faced them. Examples may include various incarnations of the Frankenstein's Monster along with Asriel Dreemurr from Undertale.
Not good with Rejection: a character may have once had an acquaintance or love interest who befriended them prior to their turn to the dark side. One day, the individual either made an offer, confessed their love, or presented a certain proposal to the hero(es) which the hero(es) rejected. The individual, unable to cope with this, thus decided turned to the dark side. Generally, the worse the hero(es) reject(s) the individual, the farther that individual turns to the dark side. An example is Anakin Skywalker.
Being an Extremist: a character has well-intentioned goals for the greater good, but their methods and ultimate ends are proven to be very extreme to the point that they become something that is greatly despised, whether by those whom they know, the heroes, or themselves. Good examples of this include Kenny from The Walking Dead.
From Victim to Villain: these characters were once people who tried to be friendly and do good, but were rewarded with injustice and/or ingratitude by society. Often, they were victims of bullying or abuse, and eventually they were pushed so hard they finally snapped and now seek revenge on the very society that shunned and hated them. Despite their tragic nature, very few of these characters actually are redeemable because they almost never forgive or forget, and/or are now completely insane and cannot be reasoned with. Often, they enact the exact same cruelty, or even worse cruelty, on their tormentors, and even innocent people. An example is Akame who became merciless towards her enemies after the harsh treatments she received from the Empire in her youth.
From Sugar to Excrement: these characters were once highly revered and loved by their peers but turned evil, often by greed, bringing shock and despair to those who knew them. Rhinox/Tanko is a good example of this kind of situation.
Being Possessed/Brainwashed: once heroic, these characters were either possessed or brainwashed by another villain or corruptive presence, and consequently became villains. However, this is the rarest and most controversial form of a hero turning to the dark side since they did not turn evil by their own free will and essentially had to be programmed into being villainous. Examples may include most of the characters from the Possessed/Brainwashed category, such as Liu Kang from Mortal Kombat X and Karone from Power Rangers: In Space and Power Rangers: Lost Galaxy. Most of these characters redeem themselves after snapping out of their current state.