I am also against warfare but if Linux (or OSS in general) helps to enhance the security and realiability of weapons so that they don't kill civil people accidently then it's ok. It's not Linux or GPL what makes things good or bad - it's the people who use it.
If you wan't GPL to be non-violent then you also have to forbid GPL usage in embedded systems, because air planes, cars etc. also can kill people.
And don't forget that Linux users normally pay back to the community. Look at SE-Linux, or SE-Android, for example.
A drone is designed for un-manned flight, nothing more or less. Sure you can strap weapons to it and use it to make it easier to kill people, but you can do the same with a car. But you can also strap cameras and sensors to a drone and use them to do neat science (like these people: http://espo.nasa.gov/missions/hs3/)
Please don't misunderstand me: Weapons are bad. But if we can't avoid them then it is better to use the best option to make bad things a little better.
If you wan't GPL to be non-violent then you also have to forbid GPL usage in embedded systems, because air planes, cars etc. also can kill people.
And don't forget that Linux users normally pay back to the community. Look at SE-Linux, or SE-Android, for example.