Brill figured they'd be looking for Tristan. Murder a city, looking for important thing Tristan had= distraction so they can reconvene. I assumed that the pass was past the marshlands. The pictures seemed to be rocky, which seemed dryer. That was a miscommunication. It isn't that we don't want to go and attack the vastly superior barbarian force, it's that, realistically, we'd all die.
Scattering, especially for mercenaries that have been with your character for...a day or so seemed logical. We don't have time to form 'fuck yeah' bonds of friendship and camaraderie. Brill was acting on righteous impulse and probably will regret that decision. Sure, he may get murdered, but he's betting they were after Tristan and his people for a reason and he'll be alive for at least a little while. He wanted people to meet at a landmark they passed so you can come up with some plan to assault a force of fifty men with eight and a tiger.
Personally, I think you may have stacked the odds a little high for us at this stage of the game. I personally want to storm in there like a badass...but you said this was Game of Thrones esque which means no radial legendary swordsmanship and incredible illogical odds. Unless we have cavalry about to spring from the stones like Raiders of the Lost Arc, we CANNOT logically win.
Unless these barbarians stab half of their number for us.
Not to be insulting, it's just how I feel. Fifty is too great a number. I'd suggest nerfoing it to thirty of the hillfolk and ten or fifteen after Brill. That way you're still running into a probable suicide that you're pretty outnumbered, but at least there's a bare glimmer of hope that an unconscious barbarian, your untrained healer, a wild animal, an untrained monk, three rogue characters, and one warrior can hack your way through them all without any casualties.
Otherwise the only logical option is to flee. As soon as the twenty chasing Brill hear their camp is under attack it will take them not but a few mins to get there. I just don't see the semi-suicidal desire to save people we (save you and Juliette) don't know are in danger.