Why did the murderer wanted to kill Douglas?

Before the men are hauled off, McMurdo speaks to them of his dangerous game. He can tell that they are angry, but over sixty more Scowrers will be rounded up and jailed. The law was never after McMurdo: Captain Marvinknew him; he joined their “infernal lodge” (317) to expose them. Perhaps some will say he is as bad as they are, but he does not care what people say. He saved Stanger’s life; he warned Wilcox and his family.

McGinty bursts out that he is a traitor; the amused McMurdo says he does not mind the epithet, and remarks that others would call him a deliverer. He will die easier knowing that he has done good.