Seems like shes "evil" because of the few times shes needed to slap her sister for being a complete idiot.
I also feel like the sister is one of the main ones who talked behind her back because she has no idea how high society works and thus had no idea how much her sister was helping her.
(As shown when shes pissed the charity got pushed back for a fancy auction...but because of the auction the older sister was able to get a bunch of rich people to donate to the charity)
I it also has to do with her power being darkness and that she knows how to control it. ╮(￣▽￣)╭ Humans tend to be evil with things they do not know of, and can also be cruel with other beings being different from the norm.
I just want to comment after reading the first chap. Because I can't get over the fact that everyone is such bloody pricks!!! Like WTF did she even do to deserve the title 'EVIL LADY'? And her parents and siblings like seriously, they were happy that the other daugther be the bride because the fave rejected it?