The killer of a Liverpool shopworker has been cleared of a murder charge, but has been found guilty of manslaughter.

Sam Harrison, 19, stabbed Mahesh Wickramasingha in the throat during a robbery at Stanley News in Huyton, Liverpool, on November 29, 2011.

Harrison was sentenced to 10 years’ detention in a young offender institution for manslaughter, but escaped the murder charge on the basis that the jury believed that he had not meant to kill his victim. He claimed that he only meant to frighten Mahesh and forgot he was carrying a knife. The judge described it as a “merciful verdict”.

Victim support group Families Fighting for Justice has appealed to the Attorney General to review the verdict and have it upgraded to murder.

During the trial, detective chief inspector Phil McEwan described the incident as “a brutal, unprovoked and senseless attack on an innocent young family man”.

He added: “Mahesh was simply going about duties at the newsagents where he worked when Harrison walked in, armed with a knife, and stabbed him fatally in the neck.”

Mahesh had moved from Sri Lanka in early 2011 and was sending wages home to support his wife and two children. The local community has raised more than £23,000 to send to Mahesh’s family in Sri Lanka.

Chief superintendent Mike Cloherty, the area commander for Huyton, said: “The reaction of local people to his death showed Mahesh had been welcomed by the community. It was heart-warming to see people who used the newsagents and knew Mahesh speak out about what had happened.”