Isn't the theme of JL:W about overcoming ego and learning the value of teamwork?
Perhaps. I and others either don't see it, see it as badly executed, or see it as much more cliche and harder to enjoy because we've seen it so many other places, and those places didn't have a 90 minute countdown.
I'd personally argue that the theme isn't developed enough to have the proper gravitas to the story, and where it is developed, very contentious characterizations are made. It *is* an exaggeration, in my opinion, to say the entire Justice League has been transformed into jerks, but they definitely focused on a much more ego-centric interpretation of the characters and highlighted character conflicts while using broader strokes for others.
For comparison, Crisis on Two Earths had a theme of duality and choice vs causality. Considering the Crime Syndicate were introduced in this film to the viewing audience, they were given a huge amount of character moments. And they were set up to be in direct opposition to the League's interactions. The first thing Wonder Woman does in the Crisis movie? Work to allay the negative impact of Batman's prank on Flash, while Superwoman is shown later clearly acting as Owlman's kinky lover and second in command and she spends the movie working against her team. Ultraman gets to act like the world's biggest and dumbest a-hole while Superman is clearly portrayed as wiser and as the most compassionate member of the league. And Batman and Owlman have numerous scenes to show their philosophical differences, culminating in their excellent last scene where Batman represents free will and taking responsibility for one's actions while Owlman becomes the ultimate nihilistic sociopath.
In contrast, War introduces Wonder Woman enjoying ice cream for the first time and making some jokes about her using her sword at inappropriate times, while Superman seems to throw the first punch in his encounter with Batman and Green Lantern, Shazam steals Cyborg's jersey, and Superman and Wonder Woman have a half-minute light flirtation that seems to center around them both being cute and powerful. It's not entirely bad characterization decisions, but there's definitely a less subtle brushwork here and you could argue the action isn't all that much better than Crisis to justify it.
And again, a lot of War's hate is going to come from the direction its perceived as taking the films. The fact that all three of the DCNAU films released thus far have neutered their villains may be seen as a pattern War started.