Why was fighting the urge so hard?

Tristan's questions were answered when he went to his home, why the devil accepted his relationship with Liliana, why his chest hadn't had a drawing on, and why his father couldn't kill Liliana.

He was supposed to kill his beloved to become the strongest but now that he wasn't after power anymore and was only after spending the rest of his life with Liliana, he couldn't back out of it. He was being compelled to kill Liliana and it was a difficult task that was why he ran to this place to avoid another attempt.

Should he tell Liliana what happened and why she should stay away from him? No, he shouldn't tell her, she'd get scared of him and that was the last thing he wanted.

Still, in thoughts about what would happen in the end, he heard someone call his name and her voice was beautiful. That was when he realized that he was missing her, he needed to kiss her and cuddle her and feel her.