The movie doesn't clearly state why, but Venom is probably afraid of Carnage because Carnage doesn't have the moral restrictions that Venom does. They are alien symbiotes. This means they need another living host to bond with if they're going to survive long-term.
This could be setting up a similar idea from the comics, where Kasady's connection to his symbiote is blood-related. This is probably why Venom is terrified of Carnage being the color red since it would have the same significance as it did in the comics.
The reason Carnage is red is because he bonded with Cletus' blood directly just like how it happened in the comics. It was slightly altered in the movie from the comics because of course.
Carnage was once a serial killer known as Cletus Kasady, and became Carnage after merging with the offspring of the alien symbiote called Venom during a prison breakout. The symbiote amplified his psychotic nature making him even less mentally stable than he had been previously, and therefore even more dangerous.
Carnage is Venom's offspring. That is why Carnage often calls him “Pops” or the like. Like a previous poster stated Venom feels responsible for Carnage and Carnage hates his “dad”.
Venom is overpowered by Carnage but provokes Barrison into using her powers to separate Carnage and Kasady. Venom devours Carnage and kills Kasady while the collapsing cathedral crushes Barrison. Later, an alive Mulligan's eyes flash blue, revealing that he somehow absorbed a part of Carnage.
While in prison, Venom produces an offspring named Carnage. (He reproduces asexually, in case you were wondering.) Venom doesn't tell Eddie about it, and Venom and Eddie escape prison. Once they're gone, Carnage bonds with Cletus.
The Venom/Spider-Man feud began years ago due to bad journalism, Spider-Man's heroism, and the mutual hatred of one angry symbiote. Though the Lethal Protector has become an anti-hero since the '90s, Venom is still widely regarded as one of Spider-Man's most well-known and iconic villains.
The son of Carnage, Toxin, was believed to be the most powerful symbiote ever created. The raw power of Toxin was so great that he was feared even by the King in Black, Knull, creator and leader of the symbiotes.
Blue Symbiote (2) (Agony): In the comics Agony was one of the symbiotes forcibly spawned from Venom alongside several others by the Life Foundation.
The idea that he would become stronger after eating Carnage seems to come from an idea that he would be absorbing Carnage's symbiote powers and properties into himself, therefore becoming stronger. However, unlike the mutant powers of Rogue, Venom is not absorbing Carnage but consuming him.
Venom uses the same black goo-like substance that makes up his mass to generate webbing to shoot. As a result, there can be too much of a good thing. If Venom uses too much webbing, it actually pulls from his own body mass, which weakens the symbiote “suit.”
Why is Carnage so powerful? The Venom symbiote eventually found Eddie and reunited with him, unknowingly leaving behind its offspring. This offspring eventually bonded to Kasady's blood through a small cut, making the Carnage symbiote have its red appearance, which resulted in Carnage being more powerful than Venom.
It could be that in the universe of the Venom movies, this type of strong bond is already known among the symbiotes to result in stronger specimens. This could explain why Venom is so fearful of facing an enemy with a ruddy hue.
Scream was the most powerful of all the symbiotes created there.
Besides communicating the apparent fact that Venom wants to eat brains — and Peter's looks tasty to Venom — Venom's hunger appears to be proof of an intrinsic attraction to Peter Parker. Now, it's critical to point out that this has never been a real thing in the comics.
Voice actor. Venom is a villain in Marvel's Spiderman. He also known as Eddie Brock. He hates Spiderman because he thinks he is the cause of all the bad luck in his life.
In the comics, Deadpool and then Spider-Man encountered the Venom Symbiote on a planet called Battleworld during the “Secret Wars” storyline. Peter Parker wore the Black Suit Spider-Man costume and brought it back to Earth, eventually rejecting it outright for fear of its sentience.
Though Venom ultimately kills Carnage at the end of the story, he's too slow to stop Carnage from awakening Knull and luring the dark god to Earth. That's the foundation for the sequel, 2020's King in Black.
Dylan Brock is the son of Eddie Brock and Anne Weying. When Anne bonded to the Venom symbiote, she somehow became pregnant with Dylan.
The original Carnage symbiote was the first offspring of the Venom symbiote, spawned when it broke Eddie Brock out of prison. The newborn symbiote bonded to serial killer Cletus Kasady, merging with his blood to become nigh-inseparable from him, and Carnage was born.
Following it giving birth to Toxin, Cletus began affectionately referring to the Carnage symbiote using female pronouns. He has more recently reverted to referring to it with male pronouns.