DreamTheater,
So if someone tried to kill you and your family, you don't think you would be justified in protecting yourself? Even if you didn't have to kill him, just knock him out so you and your family could get away safetly. That wouldn't be alright?
As clskinner said, it is a foundational maxim of Catholic moral theology, and the commandments of Christ, that we must do no evil so that good may come of it. First, we must define what evil is.
Augustine says evil is the privation of good.
The privation of good comes about by two ways;
-A turning from God and his commandments,
-A turning from God and his ordination of creation. (natural law)
Now, we have the commandment, "Thou shalt not kill." We know this does not apply to all killing, because God even commanded the Israelites to war. God cannot sin.
In any case, Thou shalt not kill has been interpreted more to mean, "Thou shalt not murder." Murder being, of course, the taking of another human life with premeditated malice. Such killings were punished under the Mosaic law.
Now, as to your example. Christ does say, we must turn the other cheek when one strikes us. This is the ideal of charity. However, many times to defend those whom we love, to check the audacity of an unjust aggressor, or to defend another legitimate claim we may have, we may resort to violence in so far as reason allows. For example, when killing an abortion doctor, the intent is to kill. The means (fighting) are neutral. Even the Lord fights his enemies, but he does so in perfect justice.
However, with saving one's life, or another's life, or property, the intent is not to kill the perpetrator, but to save one's life or whatever the thing may be. It is an extraordinary circumstance.
I think there are others on this board that can address your question with abortion doctors better, because it is difficult for me to deal with.
http://www.newadvent.org/cathen/07441a.htm
http://www.newadvent.org/cathen/01210a.htm
http://www.newadvent.org/cathen/13691a.htm