You know what I really love about this part in Jinu's Lament?
Jinu is essentially pulling an uno reverse card on Gwi-Ma.
"Now all he does is starve."
Especially as Gwi-Ma purposefully contacts Jinu when he's at his lowest point - when he and his family are starving, helpless and unable to do anything to get out of that horrendous tough spot they psychically cannot get out of.
That's when Gwi-Ma lures him into a deal which is too good to ignore - is too good to be true - and would ultimately be Jinu's downfall.
Gwi-Ma had successfully lured Jinu into a trap.
So as Jinu is singing his mockery song to Gwi-Ma, he's basically saying to him, "How does it feel like to be in the position I was in back then - starving, vulnerable and rendered helpless?"
And most importantly, that was the situation Gwi-Ma took advantage of to entice Jinu into the deal which condemned him to never-ending pain and torment. Jinu being able to express his distain for Gwi-Ma, even if through the power of irony, is very powerful.
Now while Gwi-Ma still does still have all control as we see throughout the film, Jinu is now the one offering Gwi-Ma a deal.
And just like Gwi-Ma with Jinu, its a deal Gwi-Ma cannot refuse.
Jinu will help Gwi-Ma, someone he openly despises, if he erases his memories.
Or essentially, if Gwi-Ma agrees to stop mentally tormenting him.
Its another great showing of the dynamic between Gwi-Ma and Jinu simply by showing the deals they offer:
Gwi-Ma seeks to torment and inflict pain to those he lures in through his contracts - hope shrouded in a cruel fantasy.
All Jinu sought through his deal is merely to escape the suffering Gwi-Ma has inflicted upon him. He doesn't want anything grand, he just wants to be free from torment after 400 years.
While one seeks to destroy, one seeks to escape.
Its a scene that becomes so much more powerful on rewatch because the added details you get when you understand Jinu's character and situation makes it that much more complex.