An honor student whose talent for poetry and passion for peace made him a favorite at Hunter College died yesterday after being gunned down on a Harlem streetcorner. The killer pulled a 9-mm. handgun and shot Matthew Hall, 18, once in the back after some heated words on the corner of 126th St. and Fifth Ave. shortly after 9 p.m. Wednesday, police said. Hall had just left a weekly meeting of the Universal Zulu Nation with some pals from school before he was shot about a block away. "We have no real motive," said one investigator. "We know he was...