The sentencing guidelines for murder by an offender over 18, provide a minmum time to be served of 15 years. Aggravating factors, to increase that, include that the victim was under a disability or that the offender was in a position of trust.
Mitigating factors to reduce it include that the accused was suffering from some mental disorder insufficient to reduce the offence to manslaughter (viz. diminished responsibility; a temporary disorder at the very momernt the crime was committed) ; there was no intent to kill but only to cause gbh; provocation (meaning by the victim) which falls short of a defence (this is in effect the same as 'diminished responsibility') ; and a belief that the killing was an act of mercy. Before anyone thinks that applies, they should realise that this mitigation is meant for someone who takes a life at the victim's own request because the victim is either suffering from, or the accused knows that the victim is suffering from, endless pain and will be so for an unlimited future to which death will be a merciful end, and, in the latter case, they are not able, through illness, to express their wishes
That is why I say that this man's case presents ample grounds for what is reallly the basic tariff.