How can I turn the purest and most strong willed good people to the dark side or evil path? For example the main characters Natsu and Naruto from the animes Fairy Tail and Naruto, they are one of the strongest will and purest people with a high sense of justice. So it's hard or nigh impossible to turn them evil.
In my story my main character is an evil/villain protagonist from a magic science world. There my MC is a highly intellectual scientist and a little overpowered. There are some people with a high sense of justice with strong willed people. I want to break their will and turn them into sadistic killers or just cold blooded killers without remorse for killing people, regardless of age and gender, but without damaging their personalities or something like that. But I don't know how to turn them evil, so I want some tips or ideas how to do that.