Thou Shalt Not Kill (The Ten Commandments Study Series Book 1)

Is it lawful, according to scripture to kill someone in order to defend yourself or someone else? What about the scripture that says "Thou shalt not kill?" How can a believer justify killing someone for self preservation? What about the preservation of others? At what point do the commandments "thou shalt not kill" combined with "love your neighbor as yourself" justify killing for any reason, including that of defense?