Cletus’ speech to Eddie about consequences made this troper realize something: the entire movie has the subtle theme of consequences, and more specifically mistakes coming back to haunt you. The best examples of this? Eddie’s career is in the dumps because of his actions in the last movie, his reluctance to let Venom eat people and his general attitude to Venom’s presence leads to them splitting, Mulligan’s shooting of Frances leads to her grudge toward him, Carnage is born from Venom lashing out at Cletus and inadvertently getting Eddie bitten, and the movie ends with Eddie and Venom on the run because they escaped custody and fought Carnage and Cletus. There’s many more examples, but those are just the ones that immediately stand out.
Can basic cause and effect really be the theme of a movie given how no story can exist without it?
There’s a difference between “basic” cause and effect and the theme of consequences. If it was just a one-off, albeit massive example of cause and effect (like Venom’s actions leading to Carnage), it would be easy to dismiss it as just cause and effect. But when SO MUCH of the movie uses examples of consequences to decisions, then it’s a theme, albeit a subtle one.
Cause: The Chitauri invade New York. Effect: The Avengers unite to stop them, and while they try to minimize damage and casualties, there are still innocents caught in the crossfire. Consequences: All the knock-on effects witnessed in the Netflix shows, Tony's mental state in Iron Man 3 and Avengers: Age of Ultron, the creation of Ultron, and ultimately The Sokovia Accords. As an example of the difference between Cause, Effect, and Consequences from a storytelling perspective. Let There Be Carnage also somewhat splits the idea of consequences into two parts: the consequences of your mistakes, and the consequences of your choices. If you made a choice aware of what the consequences will be, then you accepted the consequences as part of making the choice, whereas if you made a mistake, the consequences are going to be surprising and probably unpleasant.
The theme of consequences is lent more credence when you remember this interaction between Eddie and Venom as the former recklessly drives his motorcycle upon finding out about Anne's engagement:
Venom: Slow down, pal!
Eddie: Why?! You don't believe in consequences! No injuries, you fix them! No hangovers, you mend them! I couldn't even hurt myself if I tried!
The revelation that the symbiote hive mind crosses through the multiverse and causes Venom to react with instant hostility to seeing Spider-Man, despite not knowing who he is, finally explains the frequent Adaptational Villainy used in stories featuring the symbiote. On a basic and instinctive level, it remembers how Earth-616 Spider-Man treated it, back when its sentience was rather basic and it was never portrayed as corrupting Peter, and every time they meet in an alternate universe, it tries to alter Peter so that he won't reject it.
It also explains why the symbiote gets along better with Eddie than with other hosts, and gives more meaning to why Venom said he changed his mind because of Eddie in the previous movie. On some level, Venom remembers that out of all its hosts Eddie is the one it can rely on the most, and remembers how Eddie is always there after it is rejected by Spider-Man like in Spider-Man 3 and The Spectacular Spider-Man.
Why are Cletus and Carnage so at odds with each other toward the end of the movie, when their comic counterparts are so in-tune with each other that they are effectively the same being? Frances, Cletus' “light”. She was the link to Cletus’ last bits of humanity (however tenuous it was), and his primary motivation throughout the film. Because of her presence in Cletus’ life, he never became his completely homicidal and insane comic self, and would have been satisfied living however they wanted as a Bonnie and Clyde couple. With her presence, Cletus and Carnage never reached the full synchronization that they should have, and thus never reached the peak of their potential power. Ironically, if Mulligan had actually killed Frances in the prologue, Cletus and Carnage would have been the perfect match they were meant to be, and Venom and Eddie would have died along with the rest of San Francisco.
