Well, it's a genuinely hard question for Christians. But because it is so is not a reason to reject the wisdom in the Bible. (It's hard, so I have no need to find the right answer. I can do whatever I want.) The Catholic Church, as an example, only excuses killing in the face of a 'just war-' that is, when one is brutally attacked by killers and you need to defend yourself and your family - and after all other attempts at making peace have been foreclosed. That stance has much support in the Bible. The 10 Commandments are pretty strong laws for most - but it is a summary, and it doesn't mean that thoughtful,loving and God-oriented people shouldn't use their brains to try and understand what God truly wants. And is it really so horrible that the Catholic Church would try to promote peace and disdain killing, but that it makes an exception when your family is brutally attacked? You don't have to agree with that, but it's an honorable attempt to do the right thing.
Well, Jesus Christ, as an example, strongly upheld the need to follow God's laws - but also using common sense in doing so. Thus he chastized those who criticized doing good works on the Sabbath. And actually, what better way to keep the Sabbath holy than by doing good works?