Well going by
this, the judge has decided that this was manslaughter by gross negligence in the category with the lowest level of culpability. It's a very strange conviction. From my understanding, this particular category of manslaughter is normally reserved for workplace negligence. He failed in his duty of care by bludgeoning a person to death? Even accepting that this was manslaughter not murder, quoting a judge in that piece:
It seems clear which end of the scale this is. The fault here has to lie solely with the judge - he still had the power to impose a sentence all the way up to life if he had wanted to.