The symbiote has been "killed" at least twice. Once mentioned above and later when Venom, in an attempt to end the threat of Carnage once and for all, ate its spawn. However, both times they have been replaced.
Upon learning Carnage was his offspring and murdering innocent people, Venom decided it was his responsibility to stop the serial killer in his tracks. Spider-Man and Venom were able to barely defeat the mad symbiote and have teamed up many times to defeat Carnage.
When the Poison's Queen perished, the Poison Carnage disintegrated and left Kasady restored. Still in space, the Carnage symbiote that remained in his cells shielded him from his re-entry to Earth but didn't survive the journey and left Kasady burnt. Dying from his injuries, authorities claimed his body.
Thanks to interference from Spider-Man and his allies, however, the Venom symbiote returned to its host faster, and the Carnage symbiote was mostly incinerated by an airstrike. Defeated, its remaining biomass was captured by Scorn, while Cletus was taken into custody in a Quinjet.
Like Venom, Carnage is vulnerable to sound (to a much smaller degree than Venom) and heat (to a much larger degree than Venom), and is undetectable to Spider-Man's spider-sense.
By eating Carnage, Venom is effectively returning his spawn back to his former "unborn" state within Venom's mass, not taking anything he didn't already have into his cells.
Personality. Carnage is a sentient being of pure evil, shown by how moments after the symbiote was born it wreaked havoc in Prison during its escape. Carnage also has a deep hatred for its father, Venom, wanting to kill him so that he could be the only symbiote on the planet and thus unstoppable.
Carnage's fusion with the Grendel symbiote made him ridiculously strong, easily overpowering the Hulk. Carnage's full potential allowed him to tap into a force connected to the birth of the Marvel multiverse, giving him god-like power.
This meant that, while his symbiote could be “killed,” no death it suffered would stick so long as Kasady had some form of Carnage in his veins. Over the years, Carnage went through even more evolutions.
Cletus Kasady / Carnage appears as a boss in The Amazing Spider-Man 2 (1992), Spider-Man and the X-Men in Arcade's Revenge, The Amazing Spider-Man: Lethal Foes, Spider-Man and Venom: Maximum Carnage, and Venom/Spider-Man: Separation Anxiety.
Before even bonding with the alien symbiote, he was already a violent and dangerous serial killer, and unlike Eddie Brock (whose morality is affected by his symbiote), he does everything on his own volition for no reason other than to cause chaos.
Cletus survived the fall, but to his dismay, Venom ate Carnage before it could re-bond to him. As Venom picked the utterly defeated serial killer up, Cletus admitted that while he was a killer with or without the symbiote and that he really desired Eddie's friendship.
Thanos beats Carnage every time. Carnage is strong, but Thanos is in the 100+ ton strenght group, his titan biology gives him comic power and insane durability, and his tech makes beating Carnage...
While coming close to killing Spider-Man numerous times in battles since their initial encounter, Venom also began to act as an antihero of sorts, hunting down and often killing people it believed to be guilty, while attempting to protect those in danger.
If Superman was able to outmaneuver and restrain Carnage without resorting to lethal force, he would have the upper hand. However, if Carnage was able to land a blow on Superman, it could be devastating, as Carnage's symbiote gives him incredible strength and durability.
But long before Venom was one of Marvel's biggest draws, the symbiote hero fought another hero that no normal hero could ever subdue: the Incredible Hulk. In one of the stranger corners of the Marvel Multiverse, one What If...? story saw the symbiote bond with the Hulk and consuming it.
As the 1000th symbiote born in the Venom family line, Toxin was destined for incredible feats, but he was also at risk of an even more severe mental collapse than Carnage's.
Riot is the main antagonist of the 2018 film Venom. was a symbiote originating from Klyntar and an enemy of Venom. At the time of his death, he was bonded with Carlton Drake. He is Venom's older and abusive brother.
This long-haired offspring of Venom has taken a different path from her siblings, trying to be a hero. However, her origins and journey still include some very villainous moments. Scream was first introduced in 1993 in Venom: Lethal Protector #4 alongside her siblings.
Venom came to empathize with Eddie's status as a "loser", eventually deciding to turn on its own kind and save the Earth from being invaded. The symbiote ultimately agreed to work alongside Eddie as a vigilante, to only hurt bad people despite its own confusion of how it can tell the difference of who is good or bad.
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.
Seemingly killed by the Sentry, Carnage eventually returned more powerful than ever and began harboring megalomaniacal ambitions of conquest and godhood. Cletus perished while separated from his symbiote, but a cult worshiping the symbiote god Knull bonded him to a new symbiote linked to the dark god, reanimating him.
