This article sums up the evil that horribly killed two American soldiers. It also explains why we have to formally return to outlawry. Right now the Geneva Conventions handle outlawry as the default. If someone meets specific conditions (peaceful civilian, GC-abiding soldier) he gets the protections of that part of the GCs. If not, no protection, and he can be shot out of hand like US troops did to German infiltrators at the Battle of the Bulge. But that clear rule has been trampled by lawyers who insist on seeing "doubt" in all circustances and can't bear the thought of someone being deprived of their services.
So let's invert it. Set criteria for being an outlaw and then treat them as such. Torture them if it's useful, execute them if they're trouble, and treat them badly the rest of the time.