If I remember correctly, it saw him as a threat since he never stayed dead and it saw all of his deaths through the other monkeys under its control, so it caused gradual damage to his soul which caused the hallucinations and nightmares. It also intervened directly by manipulating a chucker to see if he’d stay dead that time. Which I find kinda funny because I don’t think he would have tried so hard to kill it or even at all if it didn’t cause him so much trouble. I’d recommend a reread until he kills it in chapter 27 to have a full understanding of it. Remember that any missing chapters are free on webtoon. The first little bit eluding to the hellreaver being the reason is in chapter 10.

When did he kill a hellreaver??