To save their daughter and hand over the robbed money to Zwick, Beth and Miller drives to the location given by Zwick. On the way, Miller gets a SMS from April giving the location where their daughter is held.
Miller tells Beth to save the daughter and goes alone to confront Zwick to give him back the money and take back the daughter.
Since Zwick does not know that April has told Miller of his daughters location, Miller could have easily shot Zwick as soon as he arrives instead of surrendering to him.