They both messed up pretty badly so they are dealing with the consequences
They did. But I'm talking about from where it all started. And it all started bcz MLwas going behind her back to exact her revenge pn behalf of her without her instructions or involvement. Before that moment she was starting accept her feelings for him and the fact that their relationship could something more. But after he killed Micheal without her knowledge, you could say it fucked her up to not be able take her own revenge. So, it all kinda started bcz Willhelm took the sweet revenge from her who which could've ultimately made her peaceful.
I understand he wanted revenge as well. Then he should've had his revenge and not take hers on her behalf. I'm not saying she's blameless. But this could've given her a peace of mind she was praying for 2 lifetimes. This whole thing got fucked up from there. So, he is at fault too.