Life is not precious. It is the thoughts and the actions that come from the thinking of that which is alive that have the potential to be precious.
This fact should be at the heart of all our discussions regarding birth control and euthanasia rather than the sanctity of life per se.
Life itself has no worth. It is without value. Our actions, which are informed by our thoughts, alone have value - all our actions. Therefore, we should be mindful to train our thoughts to produce actions of positive value rather than negative value.
We are justified by our actions and our actions are justified by our belief (however you may define it) in God. This is because God is that against which our actions are valued at the same time as being that which values our actions.