Or on a gardener's fence : Tresspassers will be composted!
I'm not a lawyer, and know nothing of this case, apart from that written above, but seriously, the only excuses in law (and morally corectly IMHO) for killing someone is to[list=a][*]Prevent yourself from being killed, or[*]Prevent the death of another, and[*]a legally sanctioned killing, as in war or an exectution ordered by the courts[/list=a]
Now belting down stairs with a shotgun and shooting a burgalar doesn't fall into any of those categories. It wasn't reasonable force, and thus it was murder.
Belting downstairs with a shotgun, pointing it at a burgular, and shouting "Freeze scumbag", then calling the police while holding the burgular would be more appropriate. Perhaps if you buy a shotgun for the purpose of self defence, you should also buy a couple of pairs of handcuffs. (Goes well in your defence, as to intent, later as well.)
Note that you life doesn't have to be threatened in fact, you only have to prove (ie convince a jury) that you believed it to be threatened. Perhaps this guy just needed a better defence.