I suppose that it is possible that if Abby was right there by him that maybe he hadn't even seen Libby at the end of the bridge.(nearsighted) Maybe he thought she was by herself and it would be easy for him.
seeing this guy swiftly moving in on abby if thats what we see here, would certainly be sufficient incentive for libby to start filming. If hes this close, his grabbing Abby and the introduction of a gun could easily be the reason that both girls became powerless in the situation very rapidly...(.re the mystery shape thats been mentioned inside his left leg -could it possibly be an industrial meat cleaver in a protective sheath or box of some sort, as a link to nearby meatworks? These can be extremely thick , rectangular and with a large, flat surface area ..ghastly to contemplate I know and not an opinion just a thought..apologies if im rehashing old ideas ) .