It's thou shalt not murder. Killing for self defense, preservation, or the protection of others under your responsibility, from a murderer is perfectly acceptable. Just another part of the written word misinterpreted to suit a particular agenda of man, that has absolutely nothing to do with what God wills/desires.
Good words to live by, something civilizations can be built upon. Why God has to be invoked or the threat of eternal (*)(*)(*)(*)ation has to be emphasized to keep people on the straight and narrow path to moral living is beyond me. I blame bad parenting as the cause.