At the start, there were forty bandits and three Yin Beasts. The bandit boss had brought over all his men in this dangerous operation. After all, they were facing a Yin Beast. Taking even one down needed a group of soldiers at the very least.
With that, he didn't have the leeway to leave anyone behind in their headquarters. Well, there was also the fact that he had a use for the Yin Beast corpses to further his plans.
Thus, he had brought them all. There was also the trust factor to account for in this. He didn't trust any bandit to leave them behind in the headquarters during his absence. He had a lot of valuables there that he didn't wish for anyone to come across.
They were a secret only known to him. Due to that, he brought everyone with him on this journey to annihilate their threat.
Ten minutes since the start of the high-pitched battle, a Yin Beast was killed. Fourteen bandits paid their lives to do that.