Which would have risked somebody or some people coming up the trail from the trailhead, seeing them crossing the bridge. The bridge is approximately 850 feet long, BG probably crossed it in 4 minutes, even with the condition it's in. Now marching two juveniles at gunpoint back across the bridge would carry a lot of risks, and would probably take longer (victims scared, other issues could crop up).
Walking back across an 850 foot bridge, then across a trail on the NW side of the creek to the CS, would have been a considerable distance, more than 3 times the distance from the SE end, across the creek, to the CS. Shortest distance between two points is a straight line, and the path taken would have given BG a fair amount of cover and concealment (to use a military phrase), vs. going back across a bridge, with one victim wearing a colorful shirt, no less.