No property is worth someones life. In that regard I feel he went overboard and should be punished. Unless the robbers did something to actually threaten his life in the front yard. Vigilante justice when it involves capitol punishment is a bad idea. Very few crimes in our criminal justice system warrant capitol punishement when viewed objectively. Our laws show this pretty clearly. For a person under stress in a situation like this, he's not likely to be thinking rationally and will go overboard, ala killing someone over a relatively small amount of money. No court would have sentenced those men to death, ever. What they did wouldn't have warranted it objectively. For someone thinking this is MY neighborhood, these are MY friends, this could be MY house, etc, a empathic arguement could be made for killing them, if only to himself in his own mind. That's the dangerous part about it. Having a single person dole out a capitol punishment when he may not have all the facts, be in a bad mindset for decision making etc, is a bad idea.
PS. what they are using is a well known and accepted logicall FALLACY google slipperly slope fallacy. If this happens then so does this this this this and this, no end to the madness, blah, blah, blah.