The story of the young woman is absolutely tragic. She was later hearing gunfire at night when she tried to sleep. Obviously extremely traumatized in addition to her existing health issues. To experience something so terrifying, and then go home back to a normal routine.......a difficult time for her family. My heart feels for their deep sorrow.
http://www.snopes.com/2017/10/14/survivor-las-vegas-mass-shooting-dies-suddenly/
It seems somewhat disrespectful to discuss her personal experience following her death but since you mentioned it, I believe a link to snopes is allowed. Considering her feeling was that her and a group were being chased by someone on foot, shooting at them, I'd have to say I'm still on the side of utter panic and terror of the unknown plays tricks on the human mind.
I would have to wonder -
- how the group was so fortunate that none were shot?
- wouldn't there be bullet holes in the area where they fled to?
- could the sound of gunfire - remembering other victims said they had no idea where the gunfire originated from - plus others fleeing/running behind them create a feeling of being chased, given the terror?
ETA- I would hope that the "tons of people" you refer to on SM are getting some type of trauma counselling in the aftermath of this horrible event.