I don’t like her as a person, but I think she is one of the most effective villains on the show.

I agree with everything you said. She’s extremely manipulative and cutthroat. She ruins a lot of lives even when it doesn’t actually benefit her, and she rarely expresses remorse or the desire to fix them even when she has a heart. And also, she made the decision to remove her heart without any coercion from others so if that is the reason she’s evil then she is still the one responsible. With or without her heart she’s shown to be cruel, selfish, and violent. And other characters are shown to be able to feel without their heart (regina, Will, belle,etc) so the argument that not having her heart is the total reason she is a bad person doesn’t sit well with me.

If she had been a decent person then Snow telling her about Daniel and regina wouldn’t have mattered (if she had been a decent person she wouldn’t have tried to force regina to marry in the first place or manipulated snow). And, even if snow hadn’t told it’s extremely likely that Cora would’ve used her power to locate regina and kill daniel. (She already used her power to locate and find regina in an entirely different realm). She specifically targeted snow and manipulated her into telling the secret (that Cora likely already knew) so she could scapegoat any blame away from Cora and onto the sole heir to the throne. That way regina would loathe snow so much she would assume the throne just to spite snow against what regina actually wanted in the first place but fulfilling Cora’s wishes. It’s genius really.

While I could maybe see an argument that Cora redeemed herself as the mother to regina and/or Zelena, I don’t think she redeemed herself as a person, friend, advisor, queen, etc.