The judge will have received reports on the perpetrator which will have factored into sentencing.
The major factor here will be the fact that the perpetrator was assessed by a doctor as having the mental age of a 13 year old. That will have significantly impacted on his decision making ability and impulse control.
“One punch” manslaughter cases also often attract lower sentences because there is generally not an intention to kill (albeit there is reckless disregard for life).
This is a desperately sad situation and the family of that poor man will have to live with the trauma of his death forever. I feel terribly sorry for them all. It is usually the case that the sentencing of a crime like manslaughter feels inadequate to ‘compensate’ for the loss suffered, because justice and retribution are not identical aims, and we don’t try to achieve the latter in this country.
This sentence reflects what the judge, having access to all the relevant information and considering societal aims, feels is the most proportionate and fitting outcome for the perpetrator. It can’t, and isn’t intended to, be proportionate to the grief and loss resulting from the crime.
I hope this poor man’s family and friends can find some peace moving forward.