R: We must consider if a greater injury results from telling the truth or from omitting it.
A: In this case, murder vs adultery. Are we in a position to make this determination?
Of course you misunderstood me. The quote says:
"Even the intentional suppression of truth,
by which injury may result to others, is a violation of the ninth commandment." {PP 309.3}
Notice that the quote does not say, "by which injury may result to oneself," but "by which injury may result to others". So even if Abraham might be murdered, this wouldn’t have been a valid reason for omitting the truth if by this omission injury might result to others. Besides, the omission of truth must not be equivalent to a lie (white or whatever color).
Let’s exemplify. Suppose the police is persecuting Christians. If no one is at risk, you don’t need to raise your hand and say: "Hey, I’m a Christian. You must arrest me!" But suppose you are in a group of people and the police arrives and says: "Are there any Christians here?" Keeping silent would be equivalent to saying "No," therefore the omission of truth would be a lie. Or suppose the police says, "We know there is a Christian here. If this Christian doesn’t present himself, everybody here will die." The omission of truth would result in injury to others, therefore the Christian must reveal that he is a Christian, even if this results in injury to himself.