There are a number of common misunderstandings of double effect, and one of them seems to be that if you can do something and be exculpated by double effect, then it's morally acceptable. This is certainly not true.
Consider the paradigm case for double effect. You are walking home in a very secluded place and set upon by someone trying to kill you. You fight for your life, and in so doing you twist the knife in his hand and stab him with it. Now, you are not working in any sort of genuine law enforcement capacity, so you don't have that excuse, and in general you should not go around stabbing and killing people. But this is where double effect comes in: you weren't trying to kill someone, you were trying to save your own life, which ended up involving the stabbing of another person and his consequent death. This duality of effect, what you did because you were actually trying to do it, and what you did despite not trying to do it because you couldn't help it, is the 'double effect' of the doctrine of double effect. And the basic idea here is simply that it is not "unlawful" or "blameworthy" to kill someone if you were not trying to kill them at all but only trying to save your life.
But, of course, this doesn't mean that it was morally desirable or even morally acceptable to stab and kill your attacker. It was morally understandable. It is not culpable. But it is a very bad thing, to be avoided if at all possible. The only reason it's not blameworthy here is that it wasn't, under the circumstances, avoidable. And, what is more, if you, holding the dignity of life to be such a very high thing that you refuse to kill a man even if he is trying to kill you, so that you die instead, that will sometimes be the better way. Nonculpability is generally a very low standard to aim for in the moral life. And the only thing going for your killing of your attacker is that no reasonable person can regard it as an immoral act. But it doesn't follow that it was the best thing to do. It doesn't even follow that it was a completely good thing to do -- in fact, since direct consequences are part of what makes an action completely good or not, you know it was not a completely good thing to do. It just wasn't a wrong you yourself committed. It was a terrible thing to happen. It is the sort of thing that you should try to avoid allowing to happen. It is even entirely reasonable to feel terrible because it happened. And when such things happen -- it might not necessarily be a self-defense case, but any case where double effect draws a fine line between culpability and innocence -- good and decent people often do feel terrible about them, even when recognizing that there was nothing else they could do. Double effect doesn't make it OK; it just makes it something you weren't trying to do, and that it was unavoidable in this particular case doesn't make it acceptable in itself.
What double effect really tells us is that there is a particular kind of accident, in something very like our ordinary sense of the term (although 'accident' in our sense usually assumes that it's sudden, and in the sense relevant here we can have accidents that are ongoing or slow-moving, if they are unavoidable and not what we are aiming for), that is sometimes intimately integrated with our deliberate actions. A just person trying to save himself from an attacker only deliberately tries to save himself, not kill the attacker; but sometimes it happens by accident, and sometimes the accident was unavoidable under the circumstance. These accidents-within-our-deliberate-actions are an important thing to consider. But it's important to keep in mind the fact that the killing was only non-culpable because it was accidental, because you didn't mean to do it, and accidents are non-culpable precisely to the extent they are accidents. This doesn't mean that that's the end of the story. Some things are very serious even if they are entirely understandable accidents; they may still require that you make up for them, that you work to prevent them from ever happening again, and so on and so forth, depending on what, exactly they were. Your responsibilities don't always end at the point where you honestly say, "I didn't mean to do it," and that it was an unavoidable accident may mean you aren't guilty, but it doesn't mean that all's right with the world. But lack of guilt is all that double effect gives.
