Maze had made up his mind to avenge his master and colleagues and the injustice he felt had been done to them and had decided to do that and stop them from testifying against them, he would have to kill both Campbell and Nancy.
He contemplated the person to kill first and decided that it must be Campbell.
He was the one who had come to the slut's rescue and had called the police. He was also the one who was conscious and could do more harm to them even before they were taken to the court if law.
Nancy was already half dead though that did not mean that...