He lived in the modern world, but demons and magic became real one day. The only way to kill these things was to use a new concept called skills, and culture changed greatly. Even so, the world was destroyed more and more every day. When Riley was born in his previous world, he first held the holy sword at three years old. From then, he trained and trained to kill the enemy destroying the world. He did as he asked in his despair. He killed humans and demons. In exchange for saving one town, other people would die. When the people said they were better off without him, he left them alone, and they still died. No matter what he did, people blamed him. He died fighting the demon king, who treated him more respectfully than most of the humans he met.
Since the modern world turned into a post-Apocalyptic setting, the most emotional struggles he had were against humans themselves. Since everything went to Hell, crime and chaos became prominent. I believe he first kills a human when he is seven. His teacher raped his sister, so he eventually hunted down and killed his own teacher. His "best friend" actually plotted against him and murdered his parents, and he chose to chase and kill his best friend once he found out years later. The priestess he served cursed him to be unable to commit suicide. It sounds like a warped blessing until you learn that she was extremely corrupt and guilty of any number of crimes. By forcing him to live and obey her bidding, he witnessed and participated in atrocities under her command.
He truly saw the worst of humanity. As he died, his only goal was to rest in peace. When he was reborn against his will, his intention was to do anything to avoid living such a hateful life again. Like one of those reincarnated-into-a-villain series with MCs who try to avoid death flags, Riley does everything he can to avoid hero flags. He fulfilled his duties before, and he wants nothing more of it.
He becomes completely favored by the dragon king once he joins up with the dragons, too.
We also hardly know anything about how these two were in an "accident" that sent them to the past. Are they from a sci-fi future, where things like time travel and magic-like things are possible? Are they actually fantastical creatures that only recently developed the ability to fall through time? Are the two of them destined to be reborn together again and again? I hope the series answers these questions in the remaining chapters. We know hardly anything about their lives before falling back in time.