NUEVO LAREDO, MEXICO - When Esteban Perez sold $35 bags of heroin on the streets of this violent border city, he said he felt three things: fear, dread and terror. He feared not having enough money to bribe the local police to look the other way. He dreaded not having enough heroin left to feed his addiction. And he was terrified of not having enough cash for the drug smugglers who had sold him the narcotics and demanded a share of his profits. "I was scared of them, most of all," Perez, 24, said of the traffickers. "They ask you...