These sorcerers from the cave are all weird people with a common sense quite different from people in the large world. That explains both Zero's dealings with the kid & Mercenary and the abrupt end of the battle. Basically it's like Thirteen said: They don't give a shit about stuff not personally interesting to them. Their general ethics also seem to differ from that of the common people. That's also where you made a mistake MFauli: It seemed like a serious fight, but it obviously wasn't. There's no doubt they had similar bouts in the cave, and those also ended suddenly if the schedule so dictated.

Mercenary is the only one of the central characters with a solid head, yet he also understands he can't really understand the sorcerers. That would have made brainwashing him all the easier for Thirteen.