When Zack Snyder adapted Watchmen in 2009, he used all the tricks and tools he could to make it look like the comic book was coming to life on the big screen. However, that does not mean he was completely loyal to Alan Moore and Dave Gibbons' comic book masterpiece. There were several things Snyder changed, and the most notable was the ending of the movie; however, Snyder had his reasons for this.
In the comic, Ozymandias released a giant, genetically altered, psychic squid onto the world, killing millions of people. This did not happen in the movie. Instead, Snyder used Doctor Manhattan as the death threat against civilization.
In Snyder's movie, he did something that wasn't anywhere nearly as strange as a giant alien squid, but it achieved the same underlying goal. Here, Ozymandias planned to destroy major American cities with explosions, leaving behind the energy signatures matching Doctor Manhattan. While this changed, Rorschach and Night Owl figured out the truth like in the comics, but they were too late to stop him.
When Manhattan returns and sees what Ozymandias did in his name, he almost kills his friend, but he realizes the plan worked. President Richard Nixon declares world peace, the Doomsday Clock stops and Ozymandias saves the world by killing millions. While the endings are different, the results are the same.
In 2009, Snyder answered some questions for fans of the movie on an MTV special called Spoilers. One of the questions was about the ending's changes, and he said he wasn't worried about making changes, so the ending was not off-limits. Plus, as far as the director was concerned, it needed to be changed.
"The reason that the squid got taken out of the movie was so there'd be more Rorschach and a little bit more Manhattan," Snyder said. "Because we did the math, and we figured it took about 15 minutes to explain [the squid's appearance] correctly; otherwise, it's pretty crazy."
Snyder pointed out that the studio wanted the movie to be as brief as possible. For a director who made a four-hour Justice League, Watchmen was relatively short in comparison, but it clocked in at two hours and forty-three minutes. However, there were things that would've made the film too complicated and long, like the squid. He did have a director's cut of Watchmen, which checked in at over three hours, but he still didn't bother with the squid there.
Snyder knew that framing Doctor Manhattan would tell the same story without needing to spend extra money on creating a giant squid. Plus, introducing the squid would have been far too complex; however, it still seemed important to him to honor the Watchmen graphic novel by adding a reference to the creature.
"If you want to know about the squid — well, he makes a small appearance," Snyder said. "If you notice, [Dr. Manhattan's] reactor is actually called the Sub Quantum Unifying Intrinsic Device. You see that sign [with the S.Q.U.I.D. acronym] if you look carefully in Adrian's [lair]; it's in the consoles, and it's also behind the thing when it gets teleported."
While the giant psychic squid doesn't show up and kill everyone, the machine used at least referencea its existence. Plus, Snyder's changes had the same in-universe effect as the Squid, even if Doctor Manhattan is no alien creature.