Darren Steel claimed he had acted in self-defence and that his brother had attacked him first.
Judge Walters said that although he could not be sure of that, the attack was “vicious, sustained and seriously out of proportion of any proportion of any violence he offered you”.
The judge added it was because he “had a perception that day that someone had stolen your drugs”.
A statement read on behalf of Diane Steel, the pair’s mother, said she had been “expecting to spend the afternoon shopping with my son and I had no idea my world was about to come crashing down”.
“Every night I see Martin slumped in his chair, beaten to a pulp and covered in blood,” the statement added.
“Part of me died with Martin that day.”