[TomAnthony]

They stole a machine designed to kill others, and then used it in a reckless fashion. They are responsible, and can't use the "but my friend said" defence, I'm afraid.

If you plan to use such weaponry you have a duty to confirm whatever it is your are shooting at is the right thing. Especially if it is a known flight path for commercial airliners.

I agree with you that they likely didn't intend to shoot down a commercial, rather than military, plane but that doesn't absolve them of responsibility.