R&B star August Alsina has never heard of the phrase "Don't kick a man while he's down" ... because that's exactly what he tried to do during a brawl after a show to promote non-violence. Whoops.The show was called State Of Emergency 2 and it went down Saturday night at Chaifetz Arena in St. Louis. Proceeds from the show went to the Put Down the Pistol program. According to sources, Alsina got into it backstage with the show's promoter, Loose Cannon Slim, after Slim got upset at Alsina for showing up late. Words were exchanged and it seemed as if it would stop there ... until the punches started flying. Alsina went after Slim, who got knocked to the ground. One of Alsina's people quickly pulled him off ... while Fabolous watched from a distance -- holding his drink.