In the real world that means you don't take human life. People can debate back and forth whether it is ever moral to take a human life. Some people say it's okay when it is done to save your own life, or the lives of innocent people.
Some people believe it's wrong in all circumstances, no matter what.
Personally I believe it is justifiable when it is done to save your own life or the lives of innocent people, but I would add that the fact it may be justifiable does not change by one iota the fact that it is a terrible, terrible thing. That's also something we've discussed in the books many times. I try to have the characters behave realistically, but also morally. In the real world people like police officers or soldiers sometimes kill. Any police officer or soldier who has done this will tell you that it is an awful thing, as awful as anything can be.