A little more detail might be useful about Tony Martin.
First, he had no right to own a shotgun. As a farmer, he had been in possession of a shotgun licence but it had been withdrawn after he developed a habit of threatening passers by with the gun, and also on one occasion driving to a neighbouring farm and threatening them with it. Ergo, he had no excuse for having one in his house in the first place.
Second, the gun he had has never been a legal weapon in the UK, viz. a pump-action shotgun. Even people belonging to the now defunct gun-clubs weren’t allowed such a weapon, whereas semi-automatic rifles were (then) permitted.
Third, he had more than ‘expressed a hatred of gypsies’, he had on several occasions threatened to kill any gypsies, ‘burglars’ and any trespassers in general who he found on his property.
Fourth, doesn’t it strike you as odd that he would keep a pump-action shotgun in his bedroom? Why?
Fifth, he had booby trapped his house in an additional effort to kill ‘burglars’, to the extent that he himself had difficulty getting up and down stairs because of the treads he had removed.
Sixth, both of the burglars were wounded in the back. He did not shoot until they were already trying to get out of the house as fast as they could.
Seventh, having seen the lad who died crawling into the bushes, he then drove to (presumably another) neighbour’s property and asked to be let in, rather than call the police or the ambulance. He is clearly a monomaniac as could be seen in the interviews he gave after his sentence was reduced.
Eighth, whilst there may be many reasons why he was refused parole (we call it ‘release on licence’), the main one was, as in the US I believe, a refusal to recognise that your crime was a crime is one of the things that will deny you an early release.
Ninth, you say ‘it was dark and they were in the guy’s house’. It was dark because he didn’t switch the light on. If he’d done that, they’d probably have been driving away before he got down the stairs. So why didn’t he do it? Seems likely that it was because he wasn’t interested in protecting his home (after all, a few window locks and better security would have been cheaper than a pump-action shotgun, and more effective) but in killing somebody.
Lastly, your answer seems to suggest that you feel it is OK to deprive someone of their life because they are in your house without your permission. That, in my humble opinion, is one of the things that makes your country barbaric - notwithstanding the increase in violent crime over here. The rule in England and Wales (and I dare say Scotland) is that any violence offered by a citizen can only be defended in law if it is commensurate with the violence offered by the other party. Punch a bloke in a pub, and it’s “assault occasioning actual bodily harm”. If he punched you first, 99/100 that will excuse your subsequent blow as due to “provocation”. Pull out a stanley knife and slash his throat when he punched you first, and (if he survives) you’ll both be charged and convicted. Now isn’t that more civilised?