VQuintessence
Being "Dark" it's not the problem, it's that it feels as if you as the author, enjoy tormenting and torturing the MC. Even in places like say Warhammer 40K even the "Heroes" sometimes win, the majority of them in a Pyrrhic victory, but there are a few that are not filled with suffering. And even then it's not a problem, because those character known that it's going to be a suffering life or battle. Here the MC is lost, so it feels as if he is being tortured for sport and laughs, just like a slave.
At this point, after seeing the new stat, I would just off myself if I didn't even ask for any of this. With how much he has gone through and will go through anyone would break. When he does overcome Limitless, he would just be a husk of himself. At that point the person would just be broken and with the amount of power he would accumulate, it would just be a Massacre. Killing everything on site, out of caution, like What if it hurts him? What if it's after him? etc, etc.
This story is nowhere near dark. I cant believe ppl are actually complaining about it. What, hes lost an arm, fought mafia, been severly injured and is now being hunted by pirates? That sounds like par for the course in a world dominated by pirates. And its not like hes powerless against the circumstances. Author keep doing what your doing, dont worry about these bird brained readers who cannot appreciate a slow burn story.
I get what you're trying to get at and to be honest, almost every backstory in one piece is dark but because it's mostly depicted in a flashback readers couldn't really experience it unlike your work here where we experience it with the character. but here's my question to help me and others understand your fanfic more, are you gonna keep the will of the author Oda in this world? or is it gonna be like your work in HxH ?