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How can I develop a righteous character to an evil character?
Thanks everyone for the help1
AzureCloudSky Holy, I didn't know some of my readers would be here! It makes me really happy hehe
-Show him/her the ugly truth of the book's world
-Like other said, the mc's belief and principles slowly change
-Mc is done with bs
-Lost someone (Example: Mortal combat, Scorpion's Revenge)
-Mental Problem
-Mind Controlled?
-Used by much higher Enemy?
-?
-Im Spirit
- Edited
Know your characters goals and motivations. How they'll likely react to blocks and stakes that present certain failures.
Doing a story premise might help. I'll try to link a template.
Just say he is cold-blooded and so it is affecting his temperament after reincarnstion ( If it is in ur novel) causing a slow change and u can use some even to trigger it faster
Getting chuuni dark power and character shifted? Gradually succomb to luring of darkness, classic.
MikXL_23 Thing is, you have to realize that personality change HAS to be gradual, of course there is always that HUGE event that kinda breaks the camels back, but there should be a build up to it, as your MC is non human you could make him succumb to his instincts little by little, but there should be always minor events that show him how ugly the world is, follow by a final, Major event.
Make him do something that is against his values yet he experienced self fullfilment doing it. Then, he will be conflicted of whether to refrain or continue which will lead him to insanity because he cannot get away with the fullfiling moment every time he does it. 
Hope this helps.
Don't Force it!
If you can't find a good way to change your protagonist from righteous to villainous‚ just let it remain as it is.
And if you believe that this will be boring than choose your own unique character development path.
3000 daos all different from one another.
Righteous to noble‚ A royal being in his race that can become a human being and have adventure in the human world.
OR
Father from his race mother from human race?
Or
He suddenly acquire a Unique treasure due to said plot and become aloof from mortal feeling ....i.e he doesn't judge others based on the sentiment and thoughts of society and mortals as he would Orr already stand above them all due to said treasure .xd
MikXL_23 Generally people become evil when one of two things happen. Either they discover they enjoy evil things or they are beaten down to the point where they lose their integrity and resort to evil themselves.
If you look at the 7 deadly sins: gluttony, sloth, wrath, jealousy, lust, greed, and pride, any of those is a good starting place for the corruption of your character, or even a combination or all of them.
I'll read few chapters and give you a tip
To shift from good to evil, don't drastically switch their personality. Whatever morals they have, just make them take it further. For example, if I wanted to turn Superman into a proper villain, instead of just protecting the weak, he can take it a step further and protect the weak by eliminating all the bad guys ruthlessly. In this way, the reader still recognizes the character and understands why they transformed into their evil self.
MikXL_23 Arrogance and corruption. Make them a Mary Sue/Gary Stu that gets everything it wants and is constantly praised. Let that go to its head and give it a "since I'm so great and always right, that means everyone who says no to me or questions me is evil" mindset, essentially a god complex with ends justifying the means.
Once that's established, you can make them do horrible things without the MC itself seeming like a card-carrying villian.