Andrew Young, 40, rowed with cyclist who was riding on pavement, then his pal hit him without provocation.
Andrew Young, 40, became embroiled in an argument with Victor Ibitoye after he had told him that cycling on the pavement was dangerous.
The cyclist rode off but moments later his friend, Lewis Gill, approached Mr Young and punched him in the face without provocation.
Horrific CCTV footage shows Mr Young falling backwards by the sudden blow and striking his head on the road surface.
Callous thug Gill, 20, is then seen to walk away but not before looking back to see his victim lying motionless on the ground.
The attack happened in a busy shopping area of Bournemouth, Dorset, in broad daylight and several passersby rushed to Mr Young's aid.
Mr Young, who suffered from Asperger's Syndrome, was rushed to hospital where he died the following day from head injuries with his mother at his bedside.
Gill pleaded guilty to a charge of manslaughter and was jailed for four-and-a-half years.
Afterwards, Mr Young's grieving mother Pamela described the sentence as a 'joke'.
She said: "I saw the CCTV footage in court and you can see that Andrew didn't cause Lewis Gill any harm.
"I sat with him when he died. I wish that awful man who took my son away had pleaded not guilty so he would have got a longer sentence.
"The sentence is an absolute joke. I'm a committed Christian but I think that if someone takes a life they should be prepared to forfeit their own.
"There have been many people who have committed manslaughter or murder in this country and they never even serve a full sentence."
The incident happened at 4.25pm on November 6 last year outside a Tesco Express in Charminster, Bournemouth.
Mr Young was said to be able to speak several languages but had the social skills of a 14-year-old due to his condition.
The court heard Mr Young told Mr Ibitoye during the argument that riding his bike on the pavement was a 'dangerous activity'.
Mrs Young said: "He was very particular because of his Asperger's and he wouldn't have liked seeing someone riding a bicycle on the pavement because it was dangerous."
After the brief argument Mr Ibitoye cycled away from Mr Young and apparently had no idea what Gill was about to do.
Kerry Maylin, prosecuting, said: "The defendant claims he was worried about Andrew Young because he thought he was being menacing.
"He said Andrew Young put his hand to his pocket and he thought he was going to pull out a knife or a gun."
A post mortem examination found Mr Young had no injuries to indicate that the had tried to defend himself.
The court heard Gill punched Mr Young after the victim allegedly made a racist remark.
Steven Perian, defending, added: "He wishes he could turn back the clock and not react in the way he did."
In jailing Gill, from Sutton, Surrey, judge Keith Cutler said Mr Young did not represent a threat to him.
He said: "You are a powerfully built young man.
"You must have known that it was going to cause a significant injury and, very sadly, it did."
Gill was also sentenced to two three-month prison terms to run consecutively after committing the crime while on a suspended sentence for robbery and for handling stolen goods.
He was jailed at Salisbury Crown Court last Friday.