They were basically resigning themselves to death, find out this guy killed them all with such ease that he didn't even realize what was going on, and the guild master acts like the MC isn't cut out for this stuff and shouldn't fight anymore? Dafuq? Like I get that he was more encouraging him to not go out and doing dangerous stuff alone like that, that is reasonable, but he should have been trying to get this guy to register or find out his rank or something if he can single handily save the entire city. Use your authority to make some damn exception to the rules or something for this guy so that he's not put into a position where he can't work with you at all.
I really don't understand how no one's picked up that this kid gets fucking bored and will do whatever he want, so instead of trying to get him to sit still maybe try fucking working with him? Seriously, if he got tired of everyone trying to ground him, he could wipe you all out, but instead of trying to find some damn middle ground, like working out a plan with him, finding out what he wants, having an adventuring party travel with him (since the girls could attest that you can't form a party with him without accidentally being mind-controlled) so that he doesn't move on his own, something! This is like the 3rd time that he's gone off on his own, done something amazing (but dangerous) and they think that he's going to change his ways this time. I believe there's something commonly said about doing the same thing and expecting different results.