I have thought this myself. He could have senor heard someone on the and feared they would hear the girls so he had to abandon the idea of sexually assaulting them first and get the hell out of there as quickly and discretely as possible. I dont think he used a gun as that would have been heard but he had one that he controlled them with. I also think if he had shot them that we would have heard about ballistic evidence. He may legally carry and knew that LE could ID what kind of gun was used and would then search out legal owners and question them.
I think he brought along a stun gun which is relatively silent when compared with a firearm. He brought a weapon, imo, because his plan took into account that he was going to force his victim(s) to do as he demanded (and not to be used as a killing method, imo.)
I think he planned these murders down to the smallest details exactly as it happened. I don't think he'd bring a gun because,
if a victim tried to escape he'd have to use that gun and a gun is noisy and would draw attention... alert people to approximate location of something going on that should not be going on in public hiking trails. People are curious and, imo, some would try to figure out what was happening by heading towards the gun shot sound and some, more cautious, might call 911. Why bring a "loud announcement" weapon he might have to actually use? A knife is quiet, too, but I think there's the danger of wounding one's self "self during struggles and, because I think this was well planned, I'm assuming BG thought of this because he's aware of forensic methods, DNA, etc. So many people are in this day and age and, in some ways, it's unfortunate that they are.
Any weapon that will cause pain and disability and that's aimed at a person from close proximity is very threatening in and of itself, imo. Most people would do as they were told by a stranger directing them if they saw a stun gun. Some use direct contact, I believe, and pushed against a person's head, neck, or back from behind would cause both victims to do as told, imo. (or if he handcuffed the girls to each other once he had them under the bridge.) I guess all I can safely say is that it would be very effective on me. I would use my time after that desperately looking for any, tiny opportunity to escape