The prosecution's star witness seemingly contradicted his previous testimony that a gang of white youths left a Howard Beach party to "go kill" three blacks, saying the teens never intended to fight or kill anyone. Robert Riley, 18, testified that he was in a car that was following Michael Griffith, 23, when Griffith ran onto the Belt Parkway and was struck and killed by a car. "Everyone left there (the party) to go have a fight?" asked defense attorney Stephen Murphy, who represents one of four white youths on trial on murder and manslaughter charges in the 1986 attack. "No," Riley answered. That testimony differed dramatically from Riley's testimony last week that another defendant, Jon Lester, 18, rushed into the birthday party and shouted: "Niggers on the boulevard . . . let's go kill them."