I'm on the " same page " as you! So, let me see if I can add my two cents and various thoughts in to the mix.
Where could he have been watching from to know that they had gotten to the South end, as surely he didn't follow multiple people hoping they would go all the way to the end to trap them.
If they were on that bridge for over an hour,it seems, he did wait indeed until they got to the south end before he pounced.
If he had done this on the bridge before, I'm sure that people would recognize that by now? This was his first attempt on that bridge,and he was successful at it, SMDH.
I can't imagine him walking them all the way to the North End of the bridge under duress. My knees would give out from panic and I would not be able to walk the entire way back at gunpoint or knifepoint held on me or my good friend.
If they escaped passed him going to the north, both of them would have to escaped and pass him - I don't see that happening.
I'm still not sure if he came from the south or from the north ( leaning from South) but I think he took them off the Southeast end of the bridge.
Moo, subject to change!