And this chap is such a sweetheart that after serving his sentence, in 2017, he covered his girlfriend with petrol and flicked a lighter towards her. Now back in Prison. Has 76 offenses on his record including throwing a paving slab through somone's windscreen narrowly missing a 2 year old girl.
On the topic, the important thing is not what the person did, but what they were charged with and found guilty of.
So for the chainsaw attack:-
Dinnen of Thorgill Grove, west Hull, pleaded guilty to grievous bodily harm on Adrian Prior and affray by using threatening behaviour to cause people to fear for their safety. He also accepted charges of having an offensive weapon and criminal damage.
GBH carries a sentence of between 3 years and 16 years. The Judge must decide the offence category - in this case due to lack of premediation and a likely mental disorder (chap was on drugs) indicate lower culpability. Custody is between 3 to 5 years with a starting point of 4 years. There is then a whole raft of considerations which determine whether the starting point is increased or decreased. so serious medical conditions, admission of guilt, previous offences, mental disorder etc all play a part.
The other charges will also carry sentencing guidelines and the Judge decides whether those sentences run concurrently or consecutively. In this case I would guess that the thought was that this chap needs treating for his drug addiction and that a longer sentence is not going to be an additional deterrent.
So, you can't really compare sentencing for motorists causing fatalities with other equipment causing fatalities as the legal system is constrained by the sentencing guidelines of the offences available and which the person has been charged with.