[Livia]

The other explanation is this: He wasn't charged with murder he was charged with manslaughter. It wasn't premeditated, it was obviously a spur of the moment assault. Sentencing guidelines would be in force for the crime of manslaughter. The victim having Asberger's syndrome wouldn't have had any bearing on the sentencing, only precedent would. For instance:

Source: CPS
Single punch manslaughter cases:
R v Furby [2006] 2 Cr.App.R.(S.) 8 - guideline case
Appellant and deceased were good friends. For a reason that the court regarded as an explicable reaction, the appellant struck a single punch of moderate force to the right cheek, the deceased collapsed to the ground and died due to a subarachnoid haemorrhage. Appellant of good character. Sentence reduced to 12 months imprisonment. Case law cited confirms that if there are aggravating circumstances the sentence could be as high as four years.