Why did Gashade cut open the flour sack in "The Shooting"?

From what is available, the explanation for why he decided to leave a trail for the person following him to see was most likely discarded. Wikipedia has this from the Audio Commentary on the DVD.

According to Hellman, Eastman's script was used almost exactly as written with no need for any rewrites. However, Hellman felt the first part of the script had too much expository material involving Gashade’s trip through the desert as he returned to the mining camp, so Hellman simply deleted it, noting that “Exposition, by its very nature, is artificial.” After discarding the material, Hellman began shooting at approximately “Page 10” of the screenplay. He felt the story was “perfectly simple” and didn’t need any additional information to help the audience figure things out. Nonetheless, Corman insisted on Hellman inserting a certain amount of exposition that Corman hoped would help explain the story. Corman felt that if mention was made three times during the course of the film that Gashade had a brother, audiences would not be confused by the climactic sequence. Hellman reluctantly agreed.

So there was initially an explanation for much of the setup, but it was removed.

After seeing the movie, I personally like this review.

David Pirie in Time Out wrote, "Probably the first Western which really deserves to be called existential....Hellman builds remorselessly on the atmosphere and implications of the 'quest' until it assumes a terrifying importance in itself...What Hellman has done is to take the basic tools of the Western, and use them, without in anyway diluting or destroying their power, as the basis for a Kafkaesque drama."

There was much left open, and the movie does appear to be more about the setting and the journey we are taken on, as opposed to the plot. It is entirely possible that Willett killed the people in the town, and that although he did not know the woman, he suspected he was being followed before he cut the bag (the shots of him looking intently behind him cement that) and may have cut the bag out of guilt to lead his pursuers to him. This would also lend credibility to how open to suggestion he appears to be throughout the film. Once he has been beaten down and weakened by the journey, when he realizes this woman has been pursuing his twin brother, he expresses his shock and dismay at the final gun battle.

Now... that's just one take. Another is that the entire journey is a descent into Hell.

There really isn't a single explanation as to why the events in the film unfold the way they did. The film was created at the same time as a second western, written by the same people, using the same actors. So this wasn't a deep story that was turned into a western. This was more of a psychedelic creation being made for the sake of making a movie. That said, it is a very good movie. Not from a page-turning rendition of a story point of view, but from a cinematic viewscape point of view. Seeing the movie for what it is, rather than what we expect it should be, appears to be the point of the movie.