could someone explain the end? I'm left a bit confused and feel stupid now..

yamada is the serial killer author who's karma was to reincarnate and experience the same death as the children he murdered before. terumichi was his last victim, so in that sense they were both 'cursed' together. it ties into the fact that terumichi's life felt aimless and obsolete before he met yamada, since the entire reason he was reincarnated was to kill him for the last time. after his job is done, he can rest in peace (hence why he basically disappears at the end). and because the two are joined, it's implied that if yamada is ever reincarnated again, terumichi will be reincarnated with him

Thank you for describing it nicely,. I was confused as well