Unfortunately, that "I'll kill them." declaration is just stooping down to their level. It's not like they actively tried to kill her or even used her as a sacrificial bait. They did leave her behind, but would they have left her behind without a reason, seeing how they were in a literally deadly situation where every bit of firepower mattered? They couldn't cooperate with her, so they didn't mind her disappearing. It's a scumbag move, the proper one would have been to never let her travel with them to the dungeon in the first place, but it's not like they would have been in perfect control of their lives in that new world. A soldier can't choose his own company. Yuuka would be better off just forgetting all about those people on striking out on her own. She has got a good amount of cash, so she's not in a desperate situation, unlike Kouki & Gang.