While I get why Jayna dislikes him, it's also not this him's fault that he got manipulated in the "future-past." I'm always a tiny bit uncomfortable with taking revenge on people for things that they did under a different set of circumstances and that they didn't do. (It's such a staple of the isekai/time travel genre that it's basically impossible to avoid and I do understand that she was hurt by his actions before, but still...)
Also it's not someone's fault that they got manipulated, and Jayna understands that, like she made her relationship with her friend (who was also manipulated) stronger. But why not this guy?
I just feel bad for him, I suppose.