Also known as corruption, this is when someone previously goodhearted becomes sinister.
When someone becomes corrupted then they will most likely suffer from a tragic ending. Some stories give these people a redemption.
While there are several reasons that someone would become evil most fall under four categories. Revenge, lust, desperation, and insanity. While they have similar cores they branch out onto different themes.
This type of corruption is centered around the ideal that because someone has done something horrible to you, you deserve to do something just as bad to them. In most forms of fiction this is usually the slowest method to morph them into an evil person. It starts out slow but exponentially grows spinning out of control.
Years after he was finally able to kill Itachi but learned about the elders control in the Uchiha massacre and wanted revenge on the whole village.
Because of revenge Sasuke risked his body with his curse mark, destroyed his bonds with friends, and made himself a wanted fugitive.
Revenge commonly sends people down a dark road where they eventually become as bad, or worse, than the force they originally tried to stop.
The most temping of all corruption. This could mean a lust for power or lust for a loved one. Usually using your weakness of letting things go to lure you in. If someone becomes corrupted in this fashion they are normally led by whoever caused their corruption or has the item they want so dearly.
This form of corruption is the rarely found in literature but it is very effective form of corruption. Just like revenge it will slowly morph who you are but the goal may be different, like just trying to survive. After living through the harsh reality you realize that the world offers nothing from doing good deeds so trust no one and look out for yourself. These type of people tend to be manipulative or isolated from the world.
Desperation also includes when someone does something bad in hopes it will led to good. This is under the term of desperation because they are so desperate for peace that they will go to extreme measure that may not work.
In fiction when somebody goes insane they most likely become corrupted with their new twisted views. Before their mental instability the person was most likely good (or at least not-evil).
A prime example is Sephiroth. Originally he was a good-nature solider but when he realized that he was an experiment he lost mental stability and turned against his comrades.