This does open the wider debate about intention and as such, use of excessive force. If the gun is held illegally, as is the case here, then intention has to be questioned. It could be argued that he had avery intention of shooting anyone who might break into his property, otherwiese why woulld he need the gun? This then raises the question of premeditation and intentional use of excessive force.
I think a very good case could be argued that he should go to prison for manslaughter, the UK is not a gun owning democracy and as such the keeping of such weapons for self defence is not sanctioned, it could therefore be argued that there is no reason to possess the gun without intent to use.
As it is the prosecutors have decided not to procede along these lines but I suspect that things might be different if he were a younger man, deemed otherwise better able to defend himself.