Marvel Studios cutting Thanos' backstory worried Jim Starlin that Justice League. Joe and Anthony Russo's 2018 film had a lot riding on it; aside from bringing together the MCU as the Infinity Saga culminated, it also had to establish the Mad Titan as a worthy adversary. In the end, the blockbuster became a massive hit, but prior to its release, the prolific comic book writer was concerned that this wouldn't be the case.
Infinity War premiered on the heels of Justice League's theatrical cut. Released in November of 2017, the Warner Bros. and DC Films project was riddled with issues that continue to haunt it until now. While Zack Snyder was credited as its director, the final cut of the ensemble film was decided and assembled by Whedon, who manned extensive reshoots in post-production after its original filmmaker had to step away from it. Justice League became a total let-down, with its incoherent overall story and ineffective villain — something Starlin dreaded what would happen to Infinity War.
Speaking with Comic Book, Starlin opened up about worrying when he learned that Marvel Studios was nixing Thanos' backstory from Infinity War. While he didn't have any problems with most of the creative liberties that the Russos took for the film, watching Justice League on a plane caused him to be concerned after there was also barely any backstory for Steppenwolf in the Warner Bros. film.
Though I did have a few bad moments just before the Infinity War! I had been down to the set for the cameo shoot, and had shot ... Sat and talked with the two screen writers, Markus and McFeely and quite a bit with Joe Russo. In the Infinity War, they had had a half hour that they had to cut which was going to be Thanos's back story. There was going to be a half hour without the Avengers, apparently. So, I thought, "That's pretty cool." Then I was on a plane and I watched the Justice League movie, and they just sort of threw Steppenwolf in there at the end, and he does his thing. All in all, it was kind of a bad movie. So about a month or so before the Infinity War came out Russo said... Let me know that they had to cut the half hour of Thanos. All I could think of was, "Oh my god, that's going to make it into the Justice League movie."
In general, the Russos found a way to make sure that despite opting out of traditionally showing Thanos' backstory that his motivations were clear. Through the use of the Reality Stone in Titan, he explained what drove him to Doctor Strange. Prior to that, Infinity War showed how exactly how Gamora became his adopted daughter — something that was crucial to the story as it was the only genuine relationship the Mad Titan had in the movie. It's curious what was all in that half an hour deleted scene focusing on the villain, whether or not it tackled his connections to the Eternals and how he began his crusade. But, considering that the movie was already 2 hours and 40 minutes, including the said cut arc would've made it significantly longer.
That said, there is still an argument that it would've been best if Marvel Studios gave Thanos his own origin film. That way, Avengers: Infinity War didn't have to put much of the focus on him, allowing more screen time for the heroes, especially since a lot of them were meeting each other for the first time. While this is totally fair considering how little time the Mad Titan had properly been in the MCU, at least the film didn't end up like Whedon's Justice League like Starlin worried about.
Source: Comic Book
