Alright, after re-reading all the info I can get online, I think you're right.
"When victims of kidnapping are treated humanely or simply allowed to live, they often feel grateful and attribute positive qualities to their captors believing that they are, indeed, good people."
"Professionals have expanded the definition of Stockholm syndrome to include any relationship in which victims of abuse develop a strong, loyal attachment to the perpetrators of abuse."
What I come from is the MC is mostly I guess already have more problem with mental health, so I think it's more than just Stockholm Syndrome. (Also people throwing those words makes me doubts myself idk, I need to do research more lol)
That’s still not Stockholm Syndrome this is trauma bonding (they are not synonymous). Similar but not the same. Stockholm syndrome requires a captor and a captee no one is abducted or held hostage or forcibly removed from society in this story. Whether he’s being abused or not is a whole different discussion. The Mc obviously suffers from social anxiety or something along the lines of which isolates him while he also wishes to be loved, ma-kun is taking advantage of this weakness and implants the feeling of familiarity and kindness which the little guy has been starved of. Naturally anyone would take a liking to a person who suffices you with the thing you most desperately carved. That is not Stockholm syndrome this is simply taking advantage of someone else’s weaknesses. Stockholm Syndrome REQUIRES the victim to be forcibly isolated from the outside world.
"Stockholm syndrome is a psychological response. It occurs when hostages or abuse victims bond with their captors or abusers. This psychological connection develops over the course of the days, weeks, months, or even years of captivity or abuse."
He ain't having that. He ain't contained or kidnapped. The reason why our MC "likes" the other dude cuz he already have problem with his mental health. You can see he slowly accepting the other dude.
We get hints that he got bullied, doesn't have anyone he can trust, he always in fear, ect.
He already got lots of problem and that guy simple being affectionate makes up for his lacking affection from people around him. That guy knows this, that's why he's taking advantage of him. Maybe he will grow to love him but doesn't excuse all his wrongdoing and being literally criminal.
(I said this cuz if the 'bad guy' have positive trait people will have sympahthy towards them and its irritation cuz most of the time they don't even try to redeem and apologise for their wrongdoing or fix their past mistake.
Tbh don't look for romance in this type of manga it ain't happening. The happy ending probably MC got help and that guy got locked up.