@never_preorder
It was not really to steal the loot, that would merely be a bonus. If you reread chapter 7, at this point they're trying to escape from it.
"Defeat the tyrant and liberate us!"
As I read that line, what it appears to me is that they somehow angered it. (Or maybe their bosses made them do so intentionally) As Elysha notes, orcs can/will follow you forever. Even more so with this named Orc, so they essentially can't go any deeper in the labyrinth until they deal with it. They can sort of run away but they're at a dead end until someone kills it. (Think, It Follows, but a giant Orc) So they lure Elysha, who at level 8 is twice what's needed for the floor. Force her into combat. Either she wins and they're free, or she loses (Possibly the secondary goal of the antagonistic group) and they have to find someone else. And by chance they got Atobe, whom they saw somehow buff his merc to defeat a monster that can take down a level 3 team on its own. So more a plan they fell into to escape the "dead weight".
