First, the response is not proportional to the crime being committed. A stereo is usually worth about a couple of hundred bucks (and yes, I know that there are much more expensive stereos) and nobody should be killed over a couple hundred bucks.
Second, we live in a society governed by the rule of law. That law states that a citizen may only kill another human being when they feel that they or another person are in immediate danger of major bodily harm. Otherwise the only people authorized to deal with crime are agents of the court (cops, lawyers, judges, juries, etc.). Extra-judicial killing undermines the rule of law, which is the bedrock of our society.
As for the case itself, the car's owner just let his frustration take over and he did a stupid thing. He should have called the cops and then gone outside to hold the guy until they got there.
Extra-judicial killings are justified sometimes but from the info given in this clearly biased piece,this one wasn't a justified killing. The rule of law is undermined as badly by judicial decisions with fair regularity.
I agree, but isn't that something that would be theives should worry about, rather than their victims. Actions have consequences. One potential consequence of stealing other people's stuff should be the possiblity that they won't take kindly to your efforts and shoot your ass.
And HOW pray tell, was he supposed to do that.