If you look at the previous services posted on the church's YouTube channel, you notice that two of the "younger" men were probably standing at the front of the church (i.e., near the "altar"), playing guitar and bass and leading the musical worship. They probably got killed first, along with the guest preacher who was shot in the back as he was going up to preach. The aisles of the church--one in the center and one on each side--were narrow, and there were pews that would have made it difficult to rush the shooter. There was one exit at the back (where the shooter entered), and one small door at the front of the sanctuary. These people were effectively trapped.
I watched a few of the videos, and I didn't see many "healthy young men." It was mostly old people, a few mothers, and children. This was, above all, an act of extreme cowardice.
Yes, in an active shooter situation, you are supposed to run, hide, or fight, in that order. These victims had nowhere to run, some of the survivors did successfully hide under pews or other victims, and they were too scared (and old, young, weak, and frail) to rush the gunman.
https://www.youtube.com/channel/UCwi858rGRPg0lFFVycFvvWA