The villainess is that way because her evil family only cares about her as a political pawn, so she craves attention while being afraid to show any weakness "unbecoming of a duke's daughter", and the only way she found to tick both points was to be an evil bitch. In one chapter she basically melts from some simple headpats, because she has never got any. Once she stops being so unhinged due to getting some genuine attention from MC, it turns out she can be a competent leader when situation calls for it. She is also good at studying and apparently has talent as a lecturer.
Crow was born in the slums where the only way to avoid being butchered by sociopathic murderers is basically to become one, and he happened to be good at it. However, for all the crazy yandere piece of shit that he is, he genuinely cares about Isabella in his own way, so even at his worst he draws the line at actual violence and only uses manipulation, or in a pinch sleep drugs so she doesn't know anything even happened. Also, despite all his complaints in this chapter, he actually already is OP in real combat without any arbitrary rules, and knows how to compensate for his lack of magic.
The OG heroine and her capture targets technically don't do anything wrong, because they are opposing genuine villains (like, the duke is basically the secret boss of all mafia). It's just they are acting upon their naive sense of justice while ignoring all long term political consequences without any counter plans, the heroine has a protagonist complex and is gloating like a bitch, and above all they pissed off the wrong person.