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 r*ped his sister, so he eventually hunted down and killed his own teacher. His "best friend" actually plotted against him and mu*dered 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 su*cide. 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.

The main character is portrayed as the useless son that's uncannily talented, but doesn't put in any effort or actually utilize that talent. He's close to his mom and two close aides (Ian and Sera), and has a negative rep around most of the other family members (though his father seems to still have faith in him). He's extremely protective of his mother, and picks up the sword for the first time in awhile after his mom is poisoned. After it's revealed that it was his father's other wife that did it. She's exiled, and the main character goes out of the way to kill her instead of letting her come back for revenge.

the current world is basically his past world, just less-developed, and that a lot of past-problems are now bleeding over into this world.

(e.g.: After the fight on the square the protagonist somehow ends up in the mage tower's secret basement on top of a pile of narcotics, which is also the place that just happens to be on top of the evil laboratory where the unconscious girl he also carried in there for some reason was experimented on, and afterwards he just happens to have an emblem of a different noble house on him he could leave behind to mislead the owner, all of that without any foreshadowing.)

The MC ignores the issues till it becomes a huge mountain, then when he starts caring and says "its bothering me so im taking action" and im like "Dude you could had avoided this whole mess from the start...". I get that he doesn't want to be a Hero. You don't have to be a Hero when you can work in the Dark with no chains restricting you.: Leaves problem alone - becomes something huge - its bothersome - BAM I'm OP ill just solve problem... He pretty much left criminals walking in the street till he gets frustrated by the issue "You are bothering my day".. Yep he could had prevented it and still lived peacefully.