@Hiyoko Both MC and ML are idiots who refused to communicate and share each other's burdens in the previous life. She refused to tell him of the horrific bullying she was experiencing, so he thought it was safe to leave her by herself. As the phrase goes "no new is good news." He thought leaving her at the fortress was safer than taking her into the dungeons with himbut clearly his judgement that she was safe was wrong. He didn't want to burden her not understanding it made her feel useless
Spoiler!
according to spoilers: Apparently, he had been sending her lots of gifts that never made it to her due to the bullying and harassment ochestrated by the princess. His biggest issue in the previous life was being too preoccupied with his own inner thoughts that he missed obvious signs, so when she wasn't using any of the gifts he sent he was like "she must hate my gifts" in his head instead of asking her if she was disatistfied with his gifts and after they slept together, he was avoiding her because he felt he took advantage of her and was afriad of losing control around her again. Scared he'd hurt her, oblivious to how he was already hurting her by avoiding her. It was only after she died that he learned she was being bullied, living in squalor, and never got his gifts. He then murderized all the people who had bullied her