Rachel was very petite and fit. If it was a crime of opportunity it is possible he mistook her for a teenager or young woman, especially if he initially sighted her from afar (say, in the parking lot) and was only following her from behind. Realizing she was older could also be motive for the violence he inflicted.
I agree with others here who suspect he has family/relational ties to this town. He fled CA after his March crime. Also reminds me of the guy that killed Mollie Tibbetts - he was a seasonal worker.
As for the arm everyone is concerned about, seems obvious to me it is the vicitm's due to how small it is and level with the door handle. Old Cop gave a perfectly logical interpretation of why she closed the door on him a few pages back as defiance/anger.