The LN reason that it was Ruby wanting to be hero does kinda fits the one bullet shot only
The problem for this theory imo is
1. Ruby going down the ramp is improbable because of the cop guarding the ramp and a cab driver who was parked and was watching the whole time and neither cop nor cabby saw Ruby
2. Why ,if the motive was fame ,would Ruby’s 1st statements befote a camera be suggesting we would never know the truth and that people in high places were involved. Why risk agitating such people who could easily kill him while in jail , when it wasn’t true?