Bridge Guy’s role and RA’s involvement?

I want to start by saying that I’m following this trial from the other side of the world and have been trying to stay updated over the past few days. However, I have a few questions that I can’t quite piece together.

First of all, everyone is talking about BG along with the infamous video of him and the girls. Personally, I believe that RA is guilty, but what exactly makes us so sure that the man in the video is the one who killed the girls? It seems like the defense could argue that he just happened to pass by. The girls were also discussing which path to take down the bridge, and why couldn’t RA have claimed that he was just trying to help them down the bridge after hearing them discuss which way to go? Hence the ”guys, down the hill”. (I know this might sound far-fetched and of course assumes that there isn’t more evidence against RA.)

Also, if he ‘walked with intent’ and was out to murder, why did he choose Abby and Libby and not any of the other girls he passed? It seems riskier to go after two girls who were also filming each other than a lone girl on the trail.

Maybe these are silly questions that will be answered eventually, but if anyone has a good theory on this, please feel free to explain it to me, as I’m trying to follow the case despite the time difference.