As for the warrior who was the leader of that mercenary army, Zhao allowed him to continue commanding that army, which was now undead, as well as the undead army from the carrion swamp. The only ones he wouldn't command was Zhao's undead, Drunk and the black mages, Gamma, Shi and Jie, as well as Shadowless Night and the undead assassins.
Zhao knew that Drunk was not really a commander. His only experience was as a black mage working in a small group. But that warrior wasn't the same. He had spent his life researching the matters of war, so Zhao let him command his army of undead, freeing Drunk from the job. As for Shadowless Night, since he was proficient in assassination, he would control all the undead assassins. This plays into their greatest strengths.