If you're referring to The Decalogue, the key passage is incorrectly translated. It forbids murder, not killing. Earlier in the Torah, the death penalty was already legislated even before Noah. It is a part of the Sinaitic Covenant. Killing is permissable in war and for reasons of self-defense. But murder is regarded as the one crime that offends the Divine Image so it must be punished by death to honor the Name Of God in the World. After all He is the Supreme Judge Of Mankind.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." -Manuel II Paleologus