He had dropped lit cigarettes into a cardboard box filled with newspaper and plastic. He had told police a demon voice told him to do that.
Mr Thorp, in his victim impact statement, had said the fire had "completely destroyed" his livelihood. The judge said Mr Thorp had been concerned at Harrison's behaviour and that, "he feels in a way he's been penalised for coping with you rather than trespassing you as many other shop owners in town did".
Two weeks earlier Harrison had had an argument with Mr Thorp and the judge said: "Before you left you threatened to burn the shop and his mother's house down. [That showed] a degree of premeditation, that you made a threat and carried it out."
He said he needed to take into account "a degree of diminished responsibility" because of Harrison's mental illness, but not because of his drinking. But the judge said he didn't think the offence would have happened without both factors being present.
Harrison "owed a great deal to the people at Aspire [mental health organisation] who rescued you" after the offending.
Judge Harrop said imprisoning Harrison "would not protect the community" except temporarily. "It would likely set you back and you would present a greater risk to the public when released."
The judge also imposed 18 months supervision, including any mental health treatment deemed necessary.