I think the villainess was super yandere before the isekai switch. She had just lost everything dear to her, before the introduction of Style. So Style was like a consolation prize. And she felt like she had no control over her life. When she met Style she didn't want to lose control over something again (hence the slave contract). So, I think in her own way the original villainess thought she truly loved Style, if only for what he was replacing in her life.
Like, she still took care of Style when he was sick. Which is super weird if she actually hated him. I think she just simply wanted him for herself. Which is why she made him kill his mother and then denied him the ability to kill himself. She wanted him and his whole devotion. Very much the wrong way to go about it. Given that Style turned around and then left her for her sister, the betrayal of the last thing she had left was probably enough to completely break her and turn her into the most heretical last boss queen.
It also looks like Gilbert must've had much more influence over her as she grew up (in the original timeline) and given how nasty he's being in the current one, we can only assume it was not entirely her fault for ending up the way she did. She was a victim of circumstance and a child who never outgrew her trauma.