writing characters who corrupt themselves
We've all heard of character's who get corrupted by other people, or due to external circumstances. But what about characters who corrupt...themselves?
So how do you write a character like that?
#1. Give them a 'dark side'
These characters might be the sunniest, most friendly people you've ever met, but give them a dark side. Even if most of the characters don't see it, it lingers in the character's mind, and the subtleties in their behaviour. Their dark side has the potential to overwhelm them - but they fight it. There need to be people/events/memories that trigger this dark side :D
#2. What caused this dark side?
Painful, past trauma, or perhaps a situation that required this dark side to exist. Try to create a complex. woven backstory that seems organic and true to their dark side - the dark side is the result of their past, not the other way around. A character who lives to avenge their parents would probably be envious of their friends' parents.
What differs these characters from regular corrupted characters that their decsent to darkness is inveitable. No matter what happens, even if they 'win' - these characters are drawn to darkness of their own will. While there needs to be a 'trigger' of sorts - it doesn't need to be as dramatic as that of a 'trigger' in a corruption arc. For example, a character has to spend a few days in isolation, away from everyone who grounds them to reality. This could also serve as a trigger.
Show them being evil. Show them doing bad things. Show them hurting the people they love. Unlike ordinary corrupted characters, whose 'darkness' is not as thick, these kind of characters willingly gave into the darkness within themselves, so they spare no mercy to those around them. They don't make concessions for the people who loved them...
#5. The (possible) redemption
The only way for characters like them to get redeemed is by solving whatever caused this 'darkness' inside them in the first place. These kind of stories tend to revolve around characters who learn to let go of the past, or learn forgiveness - alternatively, it can be them resolving their trauma, or having someone who guides them to the light. It depends - and of course, not all characters who corrupt themselves need to be redeemed :P
*tokyo revengers manga spoiler chapters 200+* a good example of a character who corrupts themself, is mikey from tr. despite all the various futures takemichi has seen, mikey is always a 'bad guy' because it wasn't kisaki, or izana, or anyone who corrupted him, per se, but rather it was the 'dark impulsivity' in himself, a result of the unresolved trauma over Shinichiro, Emma, and Izana's death.