Wannabe edgelords are exactly the sort of people you'd expect to be most of the enemies in this situation, even without the fact that some of the most major goddesses are encouraging that behavior. Every single student at this school was given a superpower and told to go kill someone. It should come as no surprise that some amount of these people took a liking to using their power to do violence on others (or already had that liking in the first place).
And as for the MC's insistence on not standing out, there's still an obvious good reason for that. As strong as he is, there's no reason to believe he can take on all of the goddesses and all of their heroes if they came after him at once, and that's likely what they'll do if they figure out he's harboring a Demon Lord. The easiest way for someone to figure out he's harboring a Demon Lord is to ask themself "If this guy's so strong, why couldn't he kill his Demon Lord?", because the obvious answer is "Because he chose not to." But because our protagonist is a Good Guy(tm) he won't just abandon people when he can help them, even if it risks inviting more problems to himself. Basic hero shit.
The issue I have here is that 80% of the "heroes" we have seen are unhinged assholed, from the elitist bastards (the "professional" dudes on the previous world) all the way to psychos (in this chapter and the absorbing power dude in the previous arc).
Some of them being bad apples make sense, but not so many I feel. Let's not forget they STILL saved their worlds, even if it was kinda their job. Not being nice does not automatically mean being psychopaths.
And of course the most powerful heroes are the most unhinged bastards. That beggars belief, that the strongest would automatically be the shittiest.
As for not standing out, yes it makes sense that he wants to hide to some degree. But there can be a happy medium between "Show that I'm the strongest" and "pretend that I'm the weakest". In fact, in order not to stand out being just a bit below the middle of the pack is best, cause being rock bottom can also make you stand out, since it makes you a target.
Ultimately I don't know, maybe it feels so bad because I had higher expectations... It's not BAD, but I hoped better you know...