Is it always a Sin to Tell a Lie?

If I was earning money from this blog, it would appear that I'd have to turn over a portion of my earnings to LarryD ......I can't seem to come up with any ideas of my own - I have to reference something I've found on his blog. Last week it was my post Wowee Zowee Malawi which came about as a result of Larry's post on the country.

The subject of today's post comes from a link I found on Larry's post, Various and Sundry Sunday which leads to a post by a blogger calling himself Reginaldus. As I mentioned in my post Ujiosama Rides Again, I try to stay away from blogs written anonymously - and with the scant amount of information he provides in his profile - I'd say Reginaldus fits the bill; but, I found the subject of his post It is a sin to lie, even to Planned Parenthood interesting enough to make an exception.

In his piece, Reginaldus argues that members of the pro-life group Live Action were guilty of the sin of lying when the group secretly video taped events in Planned Parenthood centers recently. In these videos, we see Live Action members posing as pimps and prostitutes while conducting a "sting" operation in an attempt to expose the unsavory practices of some Planned Parenthood employees. The Reader's Digest condensed version of his argument is that the Live Action members lied, deceived the Planned Parenthood staffers, lying is a sin and the end does not justify the means.

Reginaldus mentions a story of a man hiding Jews in his attic during the reign of the Nazis in Germany. The Nazis come to his house and ask the question, “Are there Jews hidden in your attic?” Reginaldus believes that telling the Nazis "No" would be "......a sin against the Nazis, against the German people as a whole, and against God – it is a direct offense against the truth."

Later, a blogger named Stacy left this comment;

"I read this post and thought about it a long time. I wrote about it at my blog, Accepting Abundance. If you go there you'll see I'm a convert and a mother. If I'm honest, I have to admit that if my child were in imminent danger from an unjust aggressor I would definitely lie to protect him before I would utter the words that would assure his harm or murder. I can't help it, that's the truth. Why would that be wrong? Aren't parents supposed to protect their children? This is very confusing."

whereupon Reginaldus replied,

"I left a comment over at your blog.
Here I will only say that we have to protect our children IN THE TRUTH. Your work as a mother is a cooperation in the divine plan -- but to lie is to participate in the work of Satan.
What good would it do to gain the whole world (and save a child's life), if only to lose one's soul in the process?"

Here is where that particular argument breaks down for me. Concerning the killing of someone in self defense, the Catechism of the Catholic Church says at 2264 and 2265:

2264 Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow:

If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful.... Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's.

2265 Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.

So, my question is this - if, under certain circumstances, such as when protecting an innocent child's life against an aggressor, one can kill the aggressor, why then would lying to the aggressor for the same reason be a sin?