I wonder if WB actually kicked Snyder when he was down. From what I can tell, Snyder left JUSTICE LEAGUE. He had to. Nobody should be editing and directing a major motion picture (or Season 3 of SLIDERS) under such circumstances. His daughter killed herself, a horrific, traumatizing event that put Snyder in a terrible place.
When David Peckinpah's son, Garrett, suddenly died of meningistis, Peckinpah didn't take the time to heal or mourn; he accepted stewardship of SLIDERS in Season 3 and never recovered from his torment. He dulled it with rage, affairs, heroin and cocaine. His grief never went away, he never learned to live with it or past it and in the end, it killed him.
Snyder decided to leave. When Whedon took over, he was in an impossible situation. Asked to complete a film he didn't really agree with. I think Informant himself would note: it’s not about how choice A is right and choice B is wrong: it's instead about committing to the choices one makes with style, craft, grace and conviction. Except in Whedon's case, many of these choices had been made and he was being asked to change some but not others.
I think it is very difficult for another creator to come in and complete someone else's vision when they have a completely oppositional style, and to complete it with pieces that have already been produced. If WB didn't want Whedon to be Whedon, they should have hired someone else, gotten Adam Kane or Greg Beeman or Allan Arkush (HEROES) or promoted Snyder's director of photography -- but it's clear that they wanted the AVENGERS director to make a Snyder movie more a Whedon movie and Whedon did what he was paid to do.
From what I can tell, the parademons feeding on fear was not part of the original storyline. Instead, the plot was that the parademons could consume and assimilate human beings and turn them into parademons.
This is just conjecture based on bits and pieces of what's been leaked. I think that that in the Snyder version, Superman, still unsteady after his resurrection, would be attacked by Steppenwolf's hordes and nearly corrupted into an agent of Apokolips. But during the process, Superman would have a vision of the Knightmare future -- Lois dead, the world a devastated wasteland, Batman fighting a losing rebellion, Superman under the control of the anti-life equation -- and Superman's horror would allow him to cast off the parademon infection. Superman would defeat Steppenwolf but now be struck by a new vision of the future -- the coming of Darkseid, the fear that the anti-life equation is suppressed but not gone and could turn him into a soldier for the other side in the war to be fought in JUSTICE LEAGUE II.
But Snyder left JUSTICE LEAGUE and it became clear that he needed a clean break with the DCEU and would never direct JUSTICE LEAGUE II. Whedon was instructed to conclude without a cliffhanger within the footage Snyder had shot with limited resources for reshoots.
The best Whedon could do: he shot a new opening in the film to indicate that the parademons feed on fear, something that was not a part of the original story. Whedon wrote in a line of dialogue for Steppenwolf saying his demons were hungering to feed on the humans and their fear. Then Whedon shot the end sequence where Steppenwolf, now frightened, is attacked by his own minions and in his defeat is suddenly removed from Earth.
The parademons feeding on fear -- it's not sufficiently maintained throughout the film, existing only in the opening, one line of dialogue and then the ending. The failure to address the Knightmare sequence in the previous film is peculiar. The third Mother Box being forgotten on the roof of a car is an awkward 'fix' where the original course of events couldn't be maintained. But there's other stuff I'd defend: Bruce using the same logic he had for killing Superman to argue in favour of resurrecting him is a beautiful moment of character development.
As for the DCEU, there clearly wasn't a lot of planning. MAN OF STEEL was intended as the start to a Superman film series, not a DC universe. But it was a respectable hit instead of a global blockbuster. The thinking was adding Batman and Wonder Woman could raise earnings. The results have been mixed.
Just as JUSTICE LEAGUE fails to entirely match BVS, WONDER WOMAN is also at odds with it. BVS shows Wonder Woman claiming she walked away from humanity, but WONDER WOMAN had her inspired by it; Gal Gadot and Patty Jenkins, developing the character in their film, found they had moved in a different direction in BVS, meaning BVS had introduced Diana without a clear direction in mind. JUSTICE LEAGUE attempts to rationalize the discrepency saying Wonder Woman never abandoned humanity but avoided leadership and notoriety.
At every point in the DCEU, films have been made in an extremely improvisational fashion. BATMAN VS. SUPERMAN was re-edited to shrink Superman's role; JUSTICE LEAGUE was reworked from what was clearly a superhero horror film with jokes into more of an AVENGERS movie.