Theo came back with thousands of men behind him. Almost none of these soldiers were used in the first attack. Because they were either low-level or old. Now was the time to show themselves. They knew there weren't many troops left in the city. All they had to do was take the surrendering soldiers hostage and kill those who didn't. Theo entrusted this task to Pierce and the other nobles. He warned them several times not to pillage the city. After all, a ruined, disorderly kingdom was not what they wanted.
Their aim was to escape with minor wounds. That way, they would have enough men later on for their plans. Moreover, even these powerless capital soldiers had families in this city. Killing them would only fuel the ensuing hatred. What they needed was not loyal dogs, but siblings they could trust.