Like I said, you need to find a balance in the middle. I think all homeowners have the right to defend their property to some degree, but you can't deliberately murder them outright. The idea is to make them face the court's justice rather than your own.
To put it in a more quantitive way, i'd say anything upto GBH is acceptable, if the guy:
- Is armed with any weapon or there is good reason to suspect he he did.
- If he tried to assault anyone.
- Had a history of aggrivated burgulary, or there is an epidemic in the neighbourhood.
Anything resulting in a death, or anything very close to, should result in some kind of trial. If anyone killed a burgular inadvertly. (i.e. stabbed one in the leg, he ran off and died of bleeding later), then he shouldn't be punished. If he killed someone and it is less obvious he tried to injure someone (i.e. shot a burgular in the head, and claimed he aimed for his stomach), then he should be tried for manslaughter with deminished responsibility, with a cap on punishment. 5 years say. If someone outright tried to kill a burgular (i.e. chopped their head off with an axe, with no evidence of the burgular threatening their life), then they should be tried for murder like normal.
Of course this makes the law pretty complicated to follow, but would strike the best balance IMO. Basically, injurying burgulars = fine, killing burgulars accidently isn't fine, but should be punished sympathetically, killing a burgular deliberately (unless in self defence) isn't right.