Alleged killer was out on day pass

“His only wrong step, if you can call it that, was that he answered ‘yes,’ ’’ Assistant Suffolk District Attorney Amy Galatis said.

Yesterday, Arias stood in Boston Municipal Court, where he pleaded not guilty to killing Sierra, who grew up in Villa Victoria and planned to become a medical technician.

This type of violence has become commonplace in certain parts of Boston, where young men and even children have been slain simply because they lived or hung out in areas connected to gang conflicts. Police believe Sierra’s killing was motivated by a long-running feud between gangs in Villa Victoria and Mission Hill.

Last year, 14-year-old honor student Jaewon Martin was killed in the basketball court near Heath Street in Jamaica Plain after gang members from Roxbury decided to shoot at anyone in their rival territory.

In 2007, 18-year-old Cedirick Steele, who worked for Meals on Wheels, was fatally shot in Roxbury for the same reason.

Later that year, Steven Odom, the 13-year-old son of a preacher, was walking to his Dorchester home when he was fatally shot by a man police said was targeting gang rivals.

“Once again, an innocent child has been gunned down amid the madness of a gang feud he had no part in,’’ Suffolk District Attorney Daniel F. Conley said in a statement about Sierra. “Neither he nor his family did anything to deserve this fate. Now his name joins the terrible litany of precious young lives lost to mindless violence.’’

The killing also raised questions about why Arias was given a one-day pass. He had been placed in custody of the Department of Youth Services for a December 2010 gun possession charge, prosecutors said.

“We will absolutely be doing an internal investigation,’’ Commissioner Jane Tewksbury of the DYS said in a telephone interview. “We will do a review about every step that was taken, before, during, and after the pass.’’

Tewksbury declined to comment on Arias’s case specifically but said that juveniles are often given passes from custody that range from two hours to an entire weekend.