Imprisoned in Ryker's Island, Cletus became cellmates with Eddie Brock, who had been separated from the Venom symbiote. When the symbiote rescued Eddie, it left behind an offspring that bonded to Cletus and merged with his blood, turning him into the monstrous Carnage.

How did Venom make Carnage?

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.

Why does Carnage hate Venom?

It's part of the evil symbiotes's culture to raise the offspring with hatred. The Venom symbiote did changed quite a lot, but that's how it behaved back then, and that's why it hated its offspring.