1. MC didn’t know that the price for removing “death” so he and the ML could be together forever was his reincarnation, so the “betrayal” wasn’t done on purpose.
2. To remove “death,” ML had to be in the underworld for 300 years or so. In order not to suffer during that time (let’s assume he’d normally get tortured in the underworld), MC took his pain in the form of the bad luck that kept following him since he reincarnated.
Here are my theories:
1. MC didn’t know that the price for removing “death” so he and the ML could be together forever was his reincarnation, so the “betrayal” wasn’t done on purpose.
2. To remove “death,” ML had to be in the underworld for 300 years or so. In order not to suffer during that time (let’s assume he’d normally get tortured in the underworld), MC took his pain in the form of the bad luck that kept following him since he reincarnated.