Why were witnesses to the initial assassination in “The Parallax View” (1974) systematically killed?

Question

Rewatched Alan J. Pakula’s The Parallax View (starring Warren Beatty) this recently–first time since seeing it in 1986—and am still baffled by a key plot device.

The film starts off with an assassination of presidential candidate at Seattle’s Space Needle. It’s clearly a huge press event and tons of people see the assassination happen. The assassin—disguised as a waiter—is chased to the roof where he falls to his death. Then in the days/weeks that follow, many people—six or more—are slowly dying off as time progresses with the implication being the Parallax Corporation is behind their deaths.

Okay, I completely buy the paranoid conceit of the film: That the Parallax Corporation’s main “product” was political assassination. But it still makes no sense to me why six or more witnesses to the initial assassination had to die. Did something play out wrong during that assassination that forced Parallax to clean up it’s mess? Was something revealed that would “out” Parallax in some way?

I know that there was another waiter there—with a gun clearly shown to the viewer–who I believe would be the initial assassin’s handler. Meaning if the first “patsy” assassin choked and did not follow through, this other assassin would jump in to get the job done and possibly kill the first “failed” assassin as well. But why go through the time, energy and effort to kill at least six innocent bystanders?

Answer 1

It's possible they killed all those people to tag Warren Beatty as a conspiracy nut. The "reason" he supposedly killed the politician at the end.

Answer 2

It is a persistent trope of movies made at that time and since that a conspiracy must be unmasked by its own coverup. At least a dozen films are built around the structure that the protagonist is told about some implausible crime or wrongdoing by some implausible informant. The protagonist is disbelieving until the informant is killed, which arouses suspicious in our hero, who nonetheless remains skeptical until an attempt is made on his or her own life.

Besides this film, Capricorn One, Silkwood, and The Pelican Brief come to mind, as does the Halle Berry segment of Cloud Atlas.

I think it comes from the Kennedy assassination: people don't like to believe that history can be changed by the random acts of lunatics; imagining a conspiracy imposes structure on reality and gives as an enemy to root against. For decades, whenever someone even peripherally involved with the investigation died, the conspiracy theorists would trumpet it as further proof. When assassin Jack Ruby died after three years in prison, it was widely claimed he was poisoned, to prevent him from "talking". Once this trope, "death of a witness proves the existence of conspiracy", became fixed in the public mind, its appearance in film became inevitable.