If you look at the map I made, you can see the creek is the closest possible escape route. They can't re-cross the bridge and all the other routes of escape (indicated by red lines) are terrible choices and require running much further and, even so, not necessarily promising help of any kind. On the other hand... a ride to pick them us is going to arrive soon across the creek... safety is across the creek, imo.
A lot of us assume BG grabbed them or otherwise took control of them at the end of the bridge and "down the hill" means the hill at the end of the bridge. We have no way of knowing that to be fact. Some of us say we see a gun in BG's right pocket. Maybe the girls saw the same thing that day and didn't wait for him to get to the end of the bridge at all.
They both knew that park... I've seen a photo of Abby fishing along the creek. I've read in MSM that they both loved to go to that park. Wouldn't they know the best ways to go to get to safety? They might know the creek was shallow at that point. I bet they did, imo. The creek is approximately 0.06 mile from the creek... probably a bit more as it's hard to measure using Google Earth due to changes in elevation.
Maybe they took off running (as soon as the video Libby made was done) and crossed the creek and made it some ways up the hill just beyond where their bodies were found... with BG, all the while, running behind them. He may have caught up with them finally and said those very same words... "Guys. Down the hill." If this is true and if they did go back down that hill... that would be right where they were found in the end. Clothing could have been thrown into the river after they were deceased.
Conversely, had they made it to the top of that hill, they would have come to the cemetery and Route 300N and maybe they were hoping people would be there. What's more... down that road to the left would bring them right to the spot where their ride would be (or would soon be waiting for them) at the main entrance of the park.
It's another theory and one I've been wondering about for a long time. I think Abby and Libby thought they knew that park as well as anyone else. If you really study the maps and maps history you will see that there is only one possible good choice of where to run. Across the creek and up the hill to the cemetery, imo... all the while hoping people might be walking along the trail(s) over in that area across the creek, as well.
My entire comment is my opinion.