I agree that it does not excuse his behavior, however his behavior as now detailed by Fisher appears to primarily be a creative clash, so the studios decision to bring in Whedon is rather relevant as Fisher signed up for a completely different movie really. Which really sucks, as once he was committed to the project he couldnt just leave despite the huge changes made. If Whedon had been at the helm from the start, things may have been different as the conflict wouldn’t have arisen mid-project. Though, it seems like the studio was wrong wanting Whedon in the first place, as it appears that the audience in general always wanted the Snyder cut/version!
You can clash creatively and still be respectful. He knew what the WB wanted and agreed to do it. He shouldn't have. The WB basically wanted to compress a storyline as sprawling as Infinity War/Endgame into a sub 2hr movie. It can't be done.
As far as the quality of the final product in either case, I don't blame either Snyder or Whedon. It's on the WB and top execs there. It's hard to say what fans wanted, but I don't think fans of the DCverse wanted their franchise "Marvelized" especially when so many of the storylines used are quite similar to ones the MCU already did.