I’m not sure what you guys are arguing about but didn’t Sephiroth appear as several horrible monster-human hybrids in the game? How is appearing in the movie as a normal human that can fly, wield a sword, manipulate children, taint the Lifestream, and die after a single omnislash more powerful than his form in the game where he killed a lot of people, summoned an asteroid, required ~nine people to defeat him, and had all of Jenova or whatever?
I agree that I would have liked one more scene of introduction for Denzel. One of the few additions I like in Advent Children Complete is where Cloud decides to take Denzel home from the church. But I disagree that excluding the rest is a bad move.
This is precisely why a detailed explanation of how Sephiroth came back to life and where Kadaj, Yazoo, and Loz came from didn’t exist in the movie. It’s complicated, it’s strange, it would probably ruin the pacing, and at the end of the day, probably only diehard fans of FFVII would care about it in this detail.
As it is, the movie explains that Kadaj is a larval form of Sephiroth and his similarity to his brothers suggests that they have a similar relationship with Sephiroth. The only thing Kadaj is missing to transform is Jenova, and when he gets Jenova, he transforms. At the end of the movie, Kadaj disappears back into the Lifestream, which suggests that’s where he came from in the first place.
I think this is explanation enough when the details of how Sephiroth came back to life have nothing to do with Cloud’s story, which is what the movie is ultimately about. This isn’t lazy writing. Most authors agree that you shouldn’t info dump stuff that doesn’t matter into your story. The author needs to know all these details to create a cohesive world, but he isn’t obligated to bore his viewers with them. In this case, fans who care about the details can go read the novella in stories where those details advance the plot. What's in the movie is enough for its story… unless you want to sit there and claim that excluding boring, irrelevant explanations is bad writing.
While Advent Children is a sequel, it also functions very well at introducing newcomers to Final Fantasy. This movie was my introduction to the games, and I don’t think it would attract new people as well if it hit them with more complex details and terms from the game.
(Sorry, Sephiroth. I’m on your side. I’m just defending the movie from a different perspective.)
Just throwing this out there because it’s my goal to say it as much as possible. Kingslgaive is garbage in almost every way, and the game is lesser because Noctis was excluded from the events that take place within it.