Let’s use this analogy. Suppose the BTK killer broke into my home and I, knowing who he was, shot him to protect my family. Murder is evil but did I commit an evil act? No, because I was trying to defend myself and my family.
The same is true with lying to protect an innocent person’s life. I would lie without any qualms or even doubts that I was right.(I am thinking of extreme situations here)
It is VERY IMPORTANT to understand that a circumstance or good intention --can NOT make an intrinsically evil act into a good act. NEVER.
what then if one kills someone who is trying to kill you or your family?
it is simply NOT MURDER …the object of this is very different.
It is necessary self defense.
the object is different …when it is necessary to take someones life like this it is called self defense (or defense of ones family).
the Church is very clear about this.
From the Catechism:
1753 A good intention (for example, that of helping one’s neighbor) does not make behavior that is intrinsically disordered, such as lying and calumny, good or just. The end does not justify the means. Thus the condemnation of an innocent person cannot be justified as a legitimate means of saving the nation. On the other hand, an added bad intention (such as vainglory) makes an act evil that, in and of itself, can be good (such as almsgiving).39
1754 The circumstances, including the consequences, are secondary elements of a moral act. They contribute to increasing or diminishing the moral goodness or evil of human acts (for example, the amount of a theft). They can also diminish or increase the agent’s responsibility (such as acting out of a fear of death). Circumstances of themselves cannot change the moral quality of acts themselves; they can make neither good nor right an action that is in itself evil.
now as to the lying…
if it is a lie… then yes it is a sin. for it is a sin to lie.
of course one can use discrete language…but this may not be possible…
but in truth … God knows what we were faced with when the nazi’s came to the door…in such a horrible decision and know of what stuff we are made of (after the fall) …and this little sin (for the circumstance can lessen the evil) …while a sin it seems (if it is a lie) … will vanish in his love in a moment…
often due to our fallen nature…while trying to live one virtue …another suffers…
not that is good to lie…or that the end ‘justifies’ the means here…it does not…for one still would sin a bit…but rather the mercy of God overwhelm us…
of course like i said …it may be the case that we are not dealing with lying here…