@Qelix
I guess you can put it that way, but to be fair we don’t really know who attacked first in this situation, just that the US hunters got beaten up (at least in the manhwa, but it was implied in the novel that they got knocked out by the intense magic pressure emitted by the man, mistaking it for an attack).
That and being registered hunters most likely belonging to a renowned guild for being able to go into an A rank dungeon, I presume they do have an obligation to investigate the dungeons.
Otherwise, they’ll end up ruining their guild’s reputation for turning back from a raid just because it’s “different” or “weirder” than usual. In this situation, it’s usually more important than their own safety as it may affect the world outside.
In the end, this is all speculation, but I think it’s better to just see it as it is - a bunch of hunters from a top US guild tried to raid an A rank dungeon but was met with an unknown entity that was powerful enough to take them out.