so he's no longer hiding his identity anymore. good for him. but where the fuck were you 5 minutes ago when these bastards started stomping through and burning shit down! i know your tall lanky ass was in the village so what the fuck were you doing this whole time!
edit: if you think what i said is dumb go back to chapter 353 page 12. mephisto clearly states that he will control the perimeter on his name as a 13 crown member. that means he would protect the many ears from people coming in and doing whatever they want. and he has already confirmed he wants to make iruma the demon king, why would you all think he would just continue to test iruma even further yet. he already knows just how resolved iruma is from the beach chapter so it would make even less sense to yet again test him further.
i mean, he literally did just say that through iruma's actions vs the border patrol he was able to confirm iruma has more qualities to become a worthy king on top of proving his potential in combat by piercing the barrier despite difference in rank. i think it was pretty clear that he was still contemplating it based on similarities and differences with the delkira from his memories and hadn't fully decided to throw himself in iruma's camp until this moment
regardless, mephisto is powerful so he can kind of do whatever he wants, that's the way of the demon world as shown in this very chapter. if he doesn't think they were in danger then he didn't feel the need to step in, and he seems more pragmatic than virtuous. at the end of the day, the many ears received a fright and a small setback but that is all. i think you're somewhat blowing things out of proportion