I think this thread is a good study in human nature. The reactions to the event and his words here are telling. The problem this event highlights is much deeper than whether or not his actions were legal, and the heart of the issue has far reaching consequences for our society. This man will be prosecuted because the law criminalizes his behavior as he described it. But if we look at this case and all we take from it is "you can't do that" and don't look deeper then we are doomed to see much more of this and worse.
The issue is, we are living in a time of the destruction of the rule of law. That is the real cause of this event. Human nature being what it is means we will always have criminals. But men here agreed with each other to form laws, a government to administer those laws, and to delegate power to those authorities to ensure certain people, like the two criminals in this case, cannot use force to take what they want from others. That happy state, where men interact with each other according to mutually agreed rules rather than force, is the rule of law, and exists only as long as men have the perception that it exists.
The rule of law functions and remains as long as the authorities do what they have been delegated power from the people to do. But when that government uses that power to its own ends, to enrich and take more power from the people than they intended, to further their own selfish aims rather than the work of the people, people begin to lose faith in the rule of law and feel they must take matters into their own hands.
Consider this man. While it is possible he is just a man with no more regard for human life than the low life he shot, I doubt it. I suspect rather that he is a man that is tired of the system continually working against him rather than for him. A man embittered by the complete failure of government at large to uphold their end of the social contract. A man who watches lawlessness abound while he does his part paying taxes that are used against him as much as for him. A man who has followed the law and appealed to the authorities for help before to no effect.
When men perceive, rightly or wrongly, that the rule of law no longer exists they will invariably resort to their own measures to protect their rights and property. That leads to what we have here, a man who used his own judgment to solve the problem with no further regard to the social contract. This is a problem, whether or not his actions are morally justified, because even if this man chooses wisely the next may not, and we are right back to the anarchy and the rule of force that rule of law is intended to prevent.
When confidence in our elected representatives hovers in the single digits, a third of the country believes the president should be impeached, the majority of people believe even the judiciary is hyper partisan, and there is DAILY outrage at the behavior of the police then it is easy to conclude that the perception of the rule of law is eroding before our eyes. The only thing remarkable about this story is that it doesn't happen more often, and my prediction is that it certainly will happen more often. Anarchy, like a vacuum, will not naturally persist. People will have order, whether in contract with their neighbors through government, or through the force of their own will with weapon in hand.