@vodkavodka EDIT: Jesus this turned out way longer than intended, but reading may help you enjoy the manga more!
TL;DR I don't disagree with you and I do agree with the backstory being filled with holes, but I provide a few points and try to string them together to make the backstory feel less "artificial" and more enjoyable. Hope this helps, happy reading!
Although I do agree that the MC is definitely NOT someone I would expect to have started off THIS bad, I can still understand it. Single, drastic events like the stabbing caused a lot of pain and changed him, but not because of the event itself but rather the days after. From what I can see (and what I assume based on chapters up to 6.2), he originally perceived himself as competent as a person, a friend, a son and a brother, but the stabbing event caused the brother crumble away, his failure at the job hunts caused the son to crumble, and the isolation he forced onto himself let the friend crumble as well. All those led him to believe that he, as a person, was worthless. It appears to have been a culmination of small failures triggered by one single event that domino'd out of control. The problem is that he accepted those events as proof of his worthless nature, and began habituating it until it was unconscious. He feels worthless because he failed the people he cared about.
It's not like his family or friends would be able to easily fix this, if they could have, he would have already been fixed. I would venture a guess that the comfort and support from those he failed just make him feel worse and retreat even further, even if the intentions were kind. At this particular point, the "game" provides completely new, unrecognizable stimulus that allows him to use his kind and protective nature in a "low stakes" environment where people won't get hurt when he fails (he initially thinks it's a game). His first success inspired confidence from a place he didn't know of before, from a source that he doesn't feel guilt from, and so he runs with it because this is who he was before the spiral. Think of Anakin/Darth Vader: he could not be redeemed by his fellow Jedi because he retreated and rejected them, only by his son who was completely unknown to him and foreign, but still accepted his father and believed in Anakin's redemption.
Just like the stabbing was something completely irregular and "artificial", and led to the spiral downwards to where he was in chapter 1, so too is this "game" something irregular and "artificial" and will cause that same domino effect, except it will spiral in the opposite direction.
I understand where you are coming from, like it makes no sense how someone as guilt-ridden and kind could continue like that for 10 years to this point, but the only "artificial" part in the story that I can see (so far up to 6.2) is the stabbing which caused the initial domino. I can see how someone that is basically an upstanding guy can fall so far from this one event. What I cannot see is how the stabbing occurred and (so far) it can only be explained as "the author had to start somewhere."
Is it frustrating, annoying and even unbelievable from our perspective? Yeah! The MC is a caring, protective and loving person, how the fuck did it get this bad?? We are missing a ton of pieces IMO from his backstory (that probably should have been included but weren't so that this wouldn't get info-dumped and axed), but the pieces we were given do string along a barebones story that, at least in my opinion, do show a believable and proper backstory to have the MC reach this point. The backstory is WAY too barebones and most people wouldn't piece it together (I didn't get there until I saw your comment and started thinking), it SERIOUSLY needs more fleshing out, and readers shouldn't have to do a "MatPat Game Theory" episode just to make the MC believable, but now that I did, I can accept the MC's starting point and backstory as believable and mostly natural (the stabbing still makes no sense, same with the game but that's not backstory).
Shit this was a lot longer than I was expecting it to be, but after thinking, writing and re-reading this all, I like this manga a bit more know. I stand by what I said: the backstory is way too skeletal and makes readers throw out huge assumptions to make the situation believable. However, if this comment has helped bridge those gaps until the Author properly addresses them, then I think it is a "good enough" situation as it is only chapter 6.2 so far.
I hope this helps both you and other readers enjoy this manga a bit more, even if it is mostly my opinion and """theories""" regarding the backstory. Hopefully the Author fixes this in the next chapters, happy reading!