Kevin Jon Heller, writing on the blog Opinio Juris, recently did a great job of making the important point that while the U.S. government might be able to provide some legal justification for the continued use of drone strikes abroad, that would be different than providing a moral justification. Replying to responses on an earlier post, Heller writes: 

I still want to resist an idea that seems to underly all of the responses to my post: namely, that we cannot (or at least should not) consider collateral deaths caused by drone strikes to be immoral as long as those strikes were legal.  I strongly disagree with that idea; I think it is possible — indeed important — to insist that the drone program is profoundly immoral even if no individual drone strike ever violates the laws of war.  There is a vast philosophic literature on the difference between legality and morality, which I do not have time to discuss here. … Suffice it to say that very few people are such thoroughgoing positivists that they believe legality and morality are coterminous, even if they disagree dramatically with each other concerning the particulars of the difference. Two obvious examples: “pro-lifers” don’t consider abortion to be moral even though it is legal, while the pro-euthanasia crowd doesn’t consider assisted suicide to be immoral simply because it is almost always illegal.  Both groups simply reject the morality of the laws in question.

You can read about my views on this subject here