I am going to assume that you meant Judaism and Christianity, but the fact is that all three are Abrahamic religions. That means that they share a common history, and that they all claim to follow the 10 commandments. "Thou shalt not kill" being one of them. There are countless horror stories because this one simple rule is ignored. There is no "unless"... People can carry on waging wars, they can be in the military or police, and they can defend themselves and/or loved ones with lethal force. They just can NOT do any of these things AND claim to be a member of one of these faiths. They would be lying to themselves, and whatever God they believe in. That is why I gave up after 4 years of seminary school. I could not live with that lie.