I believe this, too. We have a positive duty to protect life from evil.
I recently wrote on FR:
...I can find no proscription of self-defense in the Bible, and I believe there is a positive duty for the preservation of life, even if it requires the use of deadly force. If you want a detailed treatise on the subject, I suggest two: Charl van Wyk's "Shooting Back: the right and duty of self-defense" and Greg Hopkins' "A Time to Kill: The Myth of Christian Pacifism".
Charl van Wyk (pronounced "Sharl Van Veek") knows of what he speaks: he defended a 1200-member church in South Africa, using only a .38-spl revolver, against multiple AK-47-wielding Communist terrorists. Then, he later went to each of the [perpetrators] in prison, told them he forgave them, and attempted to lead them to Christ.
The commandment is "thou shall not murder," not "thou shall not kill". It infers a positive duty to protect innocent life against evil. Jesus never told us to lay down and allow evil to mow down the innocent.
I have thought this through, and am willing to use deadly force in self-defense. God will judge me, perhaps, but I believe I am acting according to His Word.
Also, I would be remiss if I did not mention Massad Ayoob's book "Deadly Force: Understanding the Right to Self Defense", which briefly touches on the Christian position for self-defense, while also providing a solid legal education on self-defense.