DtA :
1) Although he'd been burgled before Mr Martin apparently did not contact the police. This is only what I've heard reported on the radio news, so I might have heard wrong.
2) If someone is convicted of murder, the only sentence which can be imposed is life. There is NO discretion on that, since it's stipulated in law. It may well be that someone sentenced to life will not be incarcerated until they pop their clogs inside.
3) A neighbour of Mr Martin's was recorded as saying ( and this isn't verbatim, but as good a paraphrase as I can remember ) that "if this hadn't happened last year then it would have happened sooner or later". Other people interviewed gave their impressions of Mr Martin's views on how to deal with burglars and other criminals, which appear to have involved a great deal of "blowing their heads off" and "machine-gunning them". Add to that, he had apparently had his fireams/shotgun licence revoked and his weapons removed, but still obtained a pump-action shotgun, a recipe for disaster.
I don't want this post to be read in any other way than that of expounding on the info mentioned by DtA, but anytime someone kills someone else the circumstances must be investigated fully by the police and it's then down to the courts and juries to decide. After all, juries are made up of the same people as you and I.