For those who aren't Catholic moral theology geeks (really, some people don't spend their spare time this way?) the "principle of double effect" goes like this: It may in certain circumstances to do something which has a result which would normally be a sin, if your object in performing this act is some other good of which the bad results are a foreseen but unintended side effect.
The classic example is, in a case of a falopian pregnancy (something which often fatal to both mother and child, and from which the child is pretty much guaranteed to die) it is possible to remove the falopian tubes in order to save the mother's life. This has the foreseen side effect of the unborn child dying, since the tubes in which the child has implanted are removed. However, it is not considered as a similar moral act to an abortion because the object is to remove the tubes and save the woman's life, not to kill the child.
Now, I think it's pretty clear to anyone that this is a useful form of moral thinking. Many acts have consequences (known or unknown) that are not what we intend. And I know that I've seen a few people (generally in mentally vulnerable times like late high school) get themselves all tied up in knots of paralyzing worry over what the unforeseen results of their actions might be.
However, just as the possession of a good hammer does not turn everything into a nail, so the possession of a fun piece of moral reasoning does not mean that it should be applied everywhere. Case in point, I recently heard someone argue that it is inherently immoral for a soldier or police officer to intentionally use "lethal force". Rather, this fellow argued, that such a person uses force in order to achieve an objective (self defense, defending another, etc.) and might as a result "accidentally" kill his opponent.
Now, I understand what he's getting at here. Standard US rules of engagement specify that shooting an armed opponent is acceptable, finishing off a successfully neutralized wounded enemy with a shot to the back of the head is not.
But I think that attempting to use the principle of double effect in this kind of situation is deeply misguided. Attempting to argue that someone who sends a number of metal slugs in the direction of another human being at over a thousand feet per second doesn't mean to do them harm is just plain unrealistic. And by making the argument, I think one fails to deal with the moral facts of the issue and makes moral theology sound foolish into the bargain.
It seems to me that the argument that should instead be made is that certain circumstances justify the taking of life -- to the extent necessary to resolve the circumstance. A soldier, policeman, or an ordinary citizen defending his family may need to use lethal force against an aggressor in order to preserve his life or the lives of others. The taking of life is justified by the situation to the extent (and not beyond the extent) necessary to achieve that objective.
In that sense, the use of lethal force is a matter of justice. However, as Christians we believe that justice and mercy must always go hand in hand. And there is indeed a place where mercy kicks in.
While justice sometimes makes the use of lethal force to achieve some good necessary, mercy emphasizes the difference between the person and the objective. The enemy who is no longer in a position to threaten you must cease to be an enemy. The justification for using lethal force is gone when the objective is achieved.
Operationally, this may not be terribly different from the double effect explanation. But I think it is much more intellectually honest and morally sound.