Thank you! Knowing that, coupled with the witnesses hearing screams, makes me wonder if this was the sequence of events.
1. PR and Libby arrive at the park. She possibly realizes she is in danger and runs out of the car.
2. PR chases her and accosts her near the river.
3. He rapes her, which is when the first witness hears the screams and leaves, with her left alive there.
4. The second set of witnesses hear screams about 10 (?) minutes later. This is Libby trying to get someone’s attention to help her.
5. In between this time and his 3rd visit to the park, she succumbs to either injuries or hyperthermia.
6. PR comes back to see if she’s still alive. He finds that she’s not and since he raped her near the river, he puts her in it and leaves.
7. Knowing he had just killed someone, he gets excited (despite what he/the defense said, his past actions were aggressive and threatening and I don’t for a second believe he wasn’t aware of that) and has to “relieve” himself again.
All of the above is IMO, I’m not stating it as fact, but it’s the most likely scenario I can come up with, working with the timeframes and the witnesses hearing the screams at different times.