Likely we'll never know what compelled Libby to begin recording... we don't know at what moment she realized they were in danger, he was dangerous, his danger would be directed at them, but such tremendous wherewithal to do it. Perhaps he just looked dangerous, upon their first encounter, which I believe was ahead of the bridge (or toward the back end of the bridge if he had crossed ahead of them, doubled back to get behind them, then tripled back, coming up from behind before G...DTH). Maybe he was walking like on a mission, head down. Maybe spoke or didn't. Mayve just looked at them in an unsettling way. Maybe he knew their names....
She could have decided to film in case SHE needed be a witness, he just looked like trouble. I mean think of it, the group of juveniles, if even one of them had captured on their phones, or BB captured him on purpose or accidentally in a photo she might have taken. Libby may have thought she'd play it later and ask her dad if he knew him....
Libby would have made for a good witness too, I believe.
What an awful set of circumstances that, just when BG was ready to abduct them, there just was no one around. One person might have made all the difference. BB on the bridge. I imagine she's a wreck, having seen the girls but not knowing they were walking into harm's way, plus having been so close herself to a murderer. That's rough emotional stuff.
One or two things different, and Abby and Libby would have made it home safely. With a weird video. Of a guy walking past them....
Would there then never have been anyone murdered at MHB? Random. Or would Abby and Libby be lured on a different day to a new place? Targeted.
The real heroism is how she kept her phone hidden. Now, he didn't know he'd been filmed (but she didn't know he didn't know) and, especially if he was aware of the factory reset and was aware her via the A_S and/or EmilyAnn45 accounts, he'd have had to reason to destroy her phone (but she wouldn't know that), so brave to keep the phone hidden. It would be their final witness.
JMO