Once he realised that he could channel his energy in a different direction, he committed to the life of an endurance athlete with such vigour that he routinely endangered his health.
There is in John McAvoy a level of competitiveness that will never allow him to be easily beaten and he is inspired by others with a similar never-say-die approach. He laughs at his own temerity when describing how - in prison - he followed an example set by Nelson Mandela, who gave up smoking on Robben Island so that the authorities wouldn't be able to punish him by removing his right to tobacco.
McAvoy professes a huge admiration for Lance Armstrong which appears to more or less factor out the cyclist's tainted history, and you sense it's not because he's indifferent about Armstrong's 'crimes' but rather that they're eclipsed by the enormity of his achievements.
These days, John McAvoy is a man on a mission. He participates in Iron Man competitions - a combination of running, cycling and swimming that is partly sport and partly self-harm.
But more important still are the inspirational talks he delivers to school children. Believing himself able to get through to troubled youngsters because he has lived the life that may be their destiny, he has become a significant evangelist for a crime-free future.
And having opted for this course, you suspect McAvoy's competitive instinct will provide him with the determination to be the best role model in the world.
You can listen to the whole the session by clicking on the audio link above, and if you contact Jessie Williams on 020 7613 3381 or via email at firstname.lastname@example.org, she'll be able to put you on the mailing list for future events, including their excellent Postcards series.