BOGOTÁ, Colombia — Gustavo Petro, a former left-wing guerrilla and the front-runner in Colombia’s presidential election next month, is promising to shake up Colombian society. And to be sure, there’s a lot that needs changing. About 40% of Colombians live below the poverty line and the country has one of the world’s largest gaps between rich and poor, according to the World Bank. “If we continue along this same path, the country will fall into the abyss,” Petro said in a recent interview with NPR.