I myself haven't made up a proper backstory for the protag to be honest. My main idea was: protag who seems to be an immoral person (for reasons i haven't fully thought of myself) is supposed to get justice at the hands of this creature (whether the creature is real or an inner manifestation his guilt is up for interpretation :>)
I didn't go into what he did because i like when the reader/player is able to fill these blanks themselves. And thank you for your lovely feedback, I really appreciate it! <3