The second part of your comment confuses me because where is any of that in the actual text? If that was the thought by the attackers then wouldn’t that mean they would have to have some deeper knowledge of rumors/goings on in the assasins guild? The thieves guild was aware of the internal struggles concerning the blackmail of the dark elf, but then also wouldn’t be afraid of the retribution by said dark elf OR consider that if the girl was weak then she may have aid from/be under direct watch of the elf at that moment?! Wouldn’t that be all the more reason to not try and mess with her?
As I mentioned in another post, considering that she would have had the foresight and skill to be able to pull of a trap that eradicates an entire guild, the phrase “just a little girl” is worse than any of the other ones that I provided because it defies all of the easily observable and unsettling degree of threat she is displaying in that moment. They are written to be absolute morons, not overconfident idiots.
Dark elf isn’t much of a threat since she’s very sick as a result of having learned 3 elements. She could fight but the strain would take years off her life and might kill her. Maybe they think she helped mc wipe out of the guild, but if so, the strain would have probably left her bedridden. plus, attacking one thieves guild means making an enemy out of every thieves guild, some of which have some descent fighters.
As for why they’re underestimating her, I do think few factors come into play. There is a very weird rivalry between scouts, thieves, and assassins (all 3 literally tell mc not to compare them to the other 2 at different points in the ln), so the thieves would’ve likely (consciously or not) downplayed the strength of the assassins and been willing to believe they fell into a trap as simple as mc blowing up their base. That’s a very assassin thing to do. The assassins also appraised mc when she first joined so the beggar could’ve known her stats from back then which are way lower than they are now.