I think you are interpreting the reason he left every one behind wrong... He left them because, at the time, he wasn't strong enough to go up against the royal family, so he retreated to the labyrinth to get stronger. He knew that he didn't have the power to get rid of everyone curse at that time, so he left. It was not because he wanted to fly solo. He is also not just fighting for the sake of his new companion, if he didn't accept the duel, it would have been a large hindrance to him. You have also misread the MC's personality. He has shown himself to actually be quite kind and caring in situations. Maybe you should try paying more attention to what is going on...
This contains a slight spoiler in paragraph 3 so skip that if you want to:
He did say that he is leaving because he lacks the power to oppose the royals however you're missing a major fact which is that he said he'll take revenge for the commander and that he lacks power to do that right now (ending of chapter 5.1). In his parting thoughts he never mentioned the other guys, he solely focused on revenge.
Also, don't put words in my mouth. I didn't say he wasn't kind. I just meant that being kind isn't a priority of his and he is pragmatic rather than emotional.
That has been his character even in their original world where he'd rather not get involved with others. It doesn't mean he is unkind, he just doesn't put effort into it and he doesn't do things to sabotage his own objective.
So my point in the original comment is that he has never put effort into socializing but now he is fighting for that girl who has proven to be very troublesome ever since they met and even after all the misunderstandings are solved, he'll take her with him to his next destination. She is no longer under any threat but he actively sought to take her along the way. In my opinion, that does not fit his profile, at least not as it was developed in the beginning. Some might call that character development but that's just one point of view. I personally don't like it.
Honestly, I loved this story for a while because it broke the isekai cliché in certain ways like the MC not being loved by everyone and thought of as a traitor and him having to run away and all but now it's back to the bullshit when this female companion of his got involved. Suddenly, the elf appearance cliché, the evil bitch sister and all this crap is too much.
I wanted this story to be somewhere between "cliché god-like MC" and "MC getting killed and taking revenge in the goriest way"....Basically, this looked like it was going to be an MC with great powers who is going to take revenge and work on his own. Isn't that why he left all the other classmates? He didn't bother trying to release the curse or look for a way to convince them or anything like that because he believed being solo from that point on was better but he suddenly got along with this stranger and is even fighting on her behalf? WTF ? where did the cool MC go ? This feels so wishy washy