Why? If it's a black and white issue then it's black and white in all circumstances.
It may not be a black-and-white issue, but your scenario certainly is.
The hypothetical you described is a group of people representing an imminent threat. The decision to shoot it down would be a no-brainer, as long as all other options were considered and rejected.
It’s already happened and TPTB made exactly that decision. A missile fired from a plane is just as lethal as an unmanned drone.
Of course, the plane’s passengers took the decision out of their hands. Just like they would, IMO, if any of our enemies ever try it again.