Murder may not have been the primary motive. If there were no other lone females, and he sees two smaller females he feels he can control, the urge to act and take them may have been a split second decision based solely on opportunity.If BG was out there stalking with the intent to kill females in broad daylight, why wasn't he more careful about being seen? Since there are more than a few who said they saw him (Holeman interview), he likely saw them, too. Also, the witness who was close enough to know his eyes were not blue.
It entirely plausible that he knocked one out to deal with only one, struck too hard and accidentally killed her. It's plausible he realized after the fact they have too much information now to be left as witnesses. He may have found the phone, killed them in anger, and mistakenly thought he did enough to destroy the phone.
He's dressed rather generically, with head covering, likely on purpose. On a small hiking path, you have to come to terms with the fact you're going to pass people, male and female. You're hoping for that lone female with enough of a distance between other hikers to see or hear nothing when you act.