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.