In a Violent Place, a Violent Life; Gangs and Guns Shaped Suspect in an Officer's Death
Shatiek Johnson is charged with second degree murder in shooting death of Police Officer Gerard Carter in Staten Island housing project on July 26; Mayor Rudolph W Giuliani and Gov George E Pataki say that 17-year-old ex-convict symbolizes failings of state's prison early release program; dysfunctional picture of Johnson that emerges from interviews with police investigators, prosecutors, neighbors and friends is of teen-ager described as once being 'nice kid,' but who by age of 15, had been convicted of manslaughter for beating death of homeless man; Johnson, in addition to Carter's fatal shooting, will be indicted in killing of rival gang leader; police and local residents, in eerie example of interconnectedness of people in neighborhood, say that Johnson and Carter were once briefly related by marriage; photo (M)