I was investigating this from the Ross Sullivan angle, but it could have been "inspiration" for anyone though I will slant toward Ross here.

First, there are some weak parallels I have heard with the movie, so I read the the wikis of the the real crime and stumbled on some interesting Zynchonicities, particularly with Ross. First, the YMCA bus trip (Ross was not on it according to the article, just his father and brother Jonathan, but it was clearly the talk of the trip) where their bus catches fire and the end up stuck in Wyoming. This is where the real "Badlands" killing spree would end a couple years later, having begun in Nebraska. This killing spree of Charles Starkweather made national news. While it is also cited as inspiration for other movies like Natural Born Killers, there is debate if the female "girlfriend" Caril Ann he dragged along on the spree was there by choice or not.

In 1960 a book came out about the main killer in the spree, Charles Starkweather. I found the cover of it fascinating.

The author’s name is Reinhardt- inspiration for rh?????? Perhaps a stretch.

And again, in typical Zynchro fashion, the last name of the first victims- Bartlett – and step parents to Caril Ann (I searched briefly for a familial connection, but the name is quite common). Maybe the name became a trigger for Ross to want a girl of his own (CJB), and eventually led to the rest.

Just some food for thought, and something about it must be important for Z to have written the letter about the 1973 movie.

-m