To be fair, the anime never made any attempt to portray Ruruka as reasonable. She was a villain, through-and-through. She was the Celeste/Mikan of the game, the mid-story antagonist.
The real problem is Izayoi. I think he's the worst character in the series. He does nothing, he has barely any interesting qualities - the only interesting thing he has said or done is say "dewicious" as an adult - he's basically a tool for Ruruka to manipulate. He's boring, and his "I still love you even though you killed me" thing isn't interesting. How neat would it have been if it turned out that he, since childhood, was her slave, and that he actually hated her? That could have been a cool twist. Perhaps her candy wore off, and Izayoi tried to kill her - making her still be a shitty person for having drugged him, but also being justified in defending herself.
I agree that Izayoi's not much better, but...
Celeste was intentionally goofy, though! I don't really see how that situation can be compared to this; even Mikan's case was better handled. Here there's multiple episodes spent on their backstory, with the show seriously/dramatically trying to say something about trust and being used and so on and... it's completely hard to take any of it seriously because it just boils down to "fuck you, eat this candy even if it kills you because ???". She's nowhere near being a character I'd call a "villain"- she's just the not important bully of a not important character and it all served only to waste time.