Terrorists are NOT "protected persons" according to the Geneva Conventions.
The marine may have violated the UCMJ, but he did not violate the Geneva Conventions. These terrorists do not carry arms openly, do not wear uniforms, fake surrenders, direct attacks at civilians, murder all of the prisoners that they capture, etc. In other words, these people (and I use the term loosely) do not adhere to any of the rules of war. They are ourside of the protection of international law. They may be shot on sight, period.