What explains cooperation between armed groups? Challenging existing literature that assumes armed groups must be similar or not cooperate at all, I argue that explicit differences are key to some cooperation. Comparative advantage explains why rebels and criminals—organizations that typically eschew collaboration—cooperate to produce violence. This article introduces “black market white labeling”—cooperation that emerges when one actor buys an illicit good or service from another and re-brands it as their own. To demonstrate this phenomenon and the conditions under which it occurs, I focus on kidnapping, an underexplored but common form of armed group violence. Drawing on 113 interviews with Colombian kidnappers and hostage recovery personnel from Colombia and the United States, I theorize the conditions under which rebels “outsource” violence to criminal gangs or produce it “in house.” This article explains the organizational dynamics of rebel-criminal cooperation that perpetuate violence against civilians.