Killing all the monster isn't really a bad idea. Sure there is the adrenaline and XP, but the only defenders right now are the girls. So killing off some of the wave isn't a bad idea. But to the point of execution, that was just dumb.
@Solaz mc noted how the shopkeeper noticed him the first time and saw her movements were similar to an adventurer. She’s likely high enough level to notice him.
@coldb7 yes, w/ that reasoning anything can be true. However, the artist already made it obvious that she's pretty fit.
Well, not explicitly said, the author mentions it in the web novel.