But Tony Martin got convicted of murder and was probably fortunate that it was reduced on appeal to manslaughter. He said before the event that he would kill burglars in future (thus he proved his pre-meditated intention to kill) and he shot the burglary in the back, so he was not being threatened. He also lied about the circumstances of the shooting.
Unless and until they make killing offenders lawful, the UK law has got it just about right.
Unfortunately people who have no legal training make unjustified assumptions based on dubious 'facts' which they read in the newspapers and others often believe what they say. Not great grounds for changing the law !