"Don't apologize. Don't feel sorry.
It is a fact of life that some men, and women, simply need killing. From the baby rapers to the violent strong who prey on the weak to the ruthless heads of state who wage war and force conflict on their neighbors. God spoke and gave the commandments to Moses whereupon He said it was against His laws to murder, not 'kill'."
I understand what you are trying to say and appreciate it. You are correct in your understanding of the prohibition against "murder" in the 10 Commandments. However, while sometimes the taking of life is "necessary", as that of a violent criminal or in warfare, we should never "enjoy" or get "satisfaction" from doing so. To enjoy it doesn't please God (read the last few verses of Jonah in the O.T. as an example). Of course, we shouldn't be riddled with guilt either. Taking life to protect innocent life is not wrong, it is correct and, I believe, God sanctioned.
A solid principle to inculcate is that "life can only legally be taken to protect innocent life in immediate danger." A theft, in of itself, does not justify the use of deadly force by the victum or others. Only if the life of the victim or others are threatened can life be legally terminated by a citizen. As an example, if that same store owner saw some punk grab a sixpack of beer and flee on foot out the door without paying, the owner would not be justified shooting him in the back as he fled (the theft of a sixpack does not justify a homocide). However, if the snatcher was obviously brandishing a weapon so that a reasonable person would feel intimidated (meaning in fear of his life), the use of deadly force is acceptable by the store owner. However, chasing him after he has left the premise and gunning him down is not considered justifiable for a citizen. If there is no immediate threat still present, homocide is not justified.
BTW - I strongly believe in the "right to carry" and have done so where legal and I felt there was a potential danger. However, I found carrying a concealed pistol to be sobering and a great responsibility - not a privilege.
I think it was Mark Twain that said, "An armed society is a polite one."
I'll say this to those who call justice an act of revenge: it is not the satisfaction brought from the vengeance; it is the satisfaction of justice being served.