The post-credits sequence of Venom introduces the audience to Cletus Kasady, a character from the Spider-Man comics who becomes the villain known as Carnage.
However, that doesn’t mean the sequel, if it happens, needs to be R-rated.
Ultimately, the feeling is that showing the violence simply isn’t necessary to the story of Carnage, because what makes the character interesting is his motivation, not his violent tendencies.
While the comics’ Eddie Brock isn’t really the nicest guy, Cletus Kasady is an actual psychopath and when he comes in contact with the alien symbiote, some pretty vile and vicious things happen.