/lit/ - Literature

I'm trying to write a character with a villanous origin, however she's in what many would say is the redemption arc since she met the love of her life that is very much fixing her broken parts of herself. My issue is: I have no idea on what or how to write the reason as to why she was previously evil without making it seem petty, or something akin to "she was evil just because". What would be something good for a start? The setting is very much fantasy, though I am not sure about the technological standpoint. I do know that there are guns, mechanisms, automatons and such available, that may just be the setting its in though.