The reason the Venom symbiote eats its victims heads is because it is eating the brain. The human brain contains a specific mineral that the symbiote needs to survive or it will slowly become insane. This is problematic because Eddy does not like the idea of being used to eat people´s brain. So in one of the comics Venom temporarily separates itself from Eddy in order to go on a feeding frenzi and cause basic mayhem. At the end of the comic Eddy finds out that the same mineral that the symbiote needs can also be found in chocolate, although it can also be found in certain fungi, and is able to contain the symbiote again.
