Responses
she's not evil for no reason, 'cause the story gives us a reason: Diana isn't my bio mom, and Katherine's bio mom doesn't like Diana and wants her dead, so Katherine does as well
I'm not saying that the reason is good, I'm saying the reason is not well thought out (kinda stupid really)
like, since Katherine isn't Dianas bio child of course she also will be evil, why must we always demonize children for their parents sins? anyways, Katherine is actually demonized now since the contract with a demon and all so c'est la vie

I wish there was a story where the fake daughter sides with her fake mother and sister, and all three plan a revenge of the adulterers
yea sure Katherine has many flaws(that we only see after she was told the truth, maybe she was a good girl before then, but too late now I guess, she's contracted some demon) but I can't help but pity her a bit since her real mother told her Diana would not love her if she knew the truth (I can even see why she'd hate Aysel since she's still alive it means what her bio mom said would likely happen)
like, she loved her mom up until she found out Diana wasn't her bio mom and now she's all up for siding with the adulterers and bulling the women who loved her enough to sacrifice her eyes? WHY? she literally could have had it good since before she knew the truth, SHE WAS ALSO A VICTIM OF THE ADULTERERS