Totally agree.
Well the dad's goal was never to teach him remorse to begin with, or if it was I missed the point completely. He wanted to destroy his world, not change his view on it. So I didn't even expect Minjae to change.
The one thing that really bothers me in what you said is the last part though. No, everyone who goes through trauma doesn't become like that. But then again we all have our own limits, different people with different minds won't feel the same, nor react the same. That it doesn't automatically lead to this result doesn't mean that it can't explain it. Every bullied kid doesn't commit suicide either, for example. But some do. "Some lived through it well" doesn't mean his circumstances should be disregarded.
Unsatisfying. Minjae didn't learn from his mistakes or experience any remorse for what he's done. He's only sad his toy got taken away. He still doesn't care about what he did to Eunsung, Eunsung's dad, Taehyun, or Sunho. A lot of people go through similar to what Minjae did as a kid and don't end up so unrepentantly cruel.