There's no way to have a non-lethal round come out of a helicopter that's a mile away. If you want something that will maintain any semblance of accuracy over a mile, it has to be high-velocity. Anything moving fast enough to be accurate over a mile isn't going to be non-lethal.
Now, you could argue that they could be close enough that they could reasonably fire an accurate shot with a non-lethal round, but that also puts them much, much closer to danger. At a mile away, they're comfortable enough that they (should) be able to take the time to assess the threat and figure out if lethal force is necessary. If they have to be close enough for a non-lethal round, I doubt they'd ever switch to it since at that range, you really want lethality if you're in a big target (i.e. a helicopter).
You shouldn't have murdered them. Killing should be the last resort, even in the heat of battle.