A man was shot in the back and killed yesterday on a street in Kensington, police said. The victim, a male in his 20s who was not immediately identified, had been shot about eight times about 2:30 p.m. in the 3300 block of F Street, police said. He was pronounced dead a half-hour later at Temple University Hospital. Police were seeking a motive for the shooting. They said that a weapon was found on the fatally wounded man.