This movie is about a moral question: can you use drones to kill terrorists even if an innocent human being will get hurt on the process?
The drones are immaterial to the question. They just happen to be the global weapon of choice to fight terrorism by some countries, such as Great Britain and the U.S. The real question is: can you kill people in self-defense, even if innocent people will get hurt? There are not very many thoughtful movies that deal with honest-to-goodness real moral problems, and most of these movies, frankly, end up evading the question by some Deus-ex-machina plot device. This movie doesn't do that, making it a superior movie in this quite limited genre of philosophical movies.
There is a distinction here that must be brought out. I think that most viewers will find that the movie tries to pit a utilitarianism principle against an intrinsic-wrongness principle. Insofar as this is how the movie will be understood by most people who watch it, I am glad that the movie slightly leans (via emotional drama) towards saying that intentionally killing an innocent human being is intrinsically wrong, notwithstanding the circumstances. Today's cultural world is so saturated with purely utilitarian principles --- which principles would say that it is morally necessary to intentionally kill an innocent human being if more than one innocent human being will be saved thusly --- that I am glad for any piece of culture that argues the contrary: there are some things that are simply wrong, and you do not do, no matter the circumstances, consequences, or price paid, and that among these are the intentional killing of an innocent human being. Hence, I am glad that most people will find in this movie a push towards the intrinsic-wrongness principle.
But a careful watching of the movie will reveal that this is not the moral dilemma being fought. The dilemma most people will find in the movie is not there at all. The real dilemma is one of "collateral damage". Yes, the phrase "collateral damage" is often abused by those who want to justify evil, but the phrase grapples in a coarse way with a real moral question, and it is with this moral question that the movie does, in fact, deal. Can an innocent human being be "collateral damage" in the use of deadly force used for legitimate self-defense?
The moral answer is, in fact, the same as the legal answer articulated in the film: it depends on necessity and proportionality. Is deadly force really necessary for self-defense, or defense of others? Can it be done otherwise? Will the number of innocent lives saved be proportionally greater than the number of innocent lives lost?
Seen in this light, the answer is fairly straightforward: if deadly force is truly necessary, if all that can be realistically done to minimize "collateral damage" of innocent human beings is actually done, and if the number of innocent lives saved is much greater than the number of innocent lives that are put in danger because of the use of force, then the use of deadly force, in itself, is a moral act. This is the principle of double effect in action, where it is permissible to undertake an action that has mainly good consequences, but some bad consequences as well, and which action is undertaken to bring about the good consequences, not the bad ones.
But, isn't this the same utilitarian principle which weighs the consequences of our actions and asks us to chooses the one with more good consequences? No. The distinction comes in when thinking about the means one uses to bring about the good ends. In the principle of double effect explicitly demands that the means by which the good is brought about be morally licit in themselves, whereas utilitarianism only cares about that the likely consequences be better than they are as a result of our actions, regardless what our actions might be. For example, a pure utilitarian would have no problem torturing the innocent children of a terrorist if the likely outcome was to save many lives; the principle of double effect would absolutely forbid such actions because it would see torture --- let alone the torture of innocent human beings --- as an immoral act in itself, and which therefore cannot licitly be the means by which good may be sought. Thus, in the movie, the distinction boils down to this: that the act envisioned is to kill terrorists, with the unfortunate (even if foreseeable) consequence of spill-over damage to innocent people; the act envisioned is not to intentionally kill innocent human beings in order to bring about good. It all hinges on where the deadly force is directed: is it directed at the innocent human being, or is it directed at the terrorists?
Which is not to say that no one in the film acted immorally! An act may be immoral not only because it is an intrinsically disordered action (such as torture), or because the act leads to (mainly) harmful consequences, but also because of the bad intentions of the actors. And it is clear in the film that the actions of the British Colonel were aimed at revenge for the death of one of her operatives. Thus, while killing the terrorists in order to protect innocent lives is a morally permissible action, the British Colonel acts immorally when the reason she orders the drone strike is revenge, regardless of the fact that everyone else in the film who cooperate with that same action acts morally.