Abel made Cain envious because God preferred the former’s sacrifice to the latter’s. Why would his sacrifice not fail? Anything Cain offered would be harvested with violence, without respect, without gratitude. Here one group of people is distancing itself from all others, and choosing to play by its own rules.
Moreover, since these were very early times and Cain represented the first agrarian communities, we can say that this civilized “hero” was the first to oppose beliefs that had always been. Ullikummi and Kumarbi, conceived later on, were the first gods to oppose God/Nature’s plan. To this end, it is safe to say that the children of Adam and Cain became the first real devil-followers, if they did not worship him directly.
At the very least these people-Adam and his descendants- turned their backs on all that was, which is why it says in the Bible they became restless wanderers. At that time there was nothing else besides Nature. What place had they?
Even if Cain is reduced to a metaphor, there is nothing that indicates anyone approving of anything Cain would have to offer. Of what benefit to anyone would it be, other than to give Cain’s progeny the feeling of having a full stomach? The tribe chose to remain outside any harmonious relationship with any creatures small or large, beholden only to itself, on account of its peculiar pipe dreams.
These people turned their backs on all the glory days of our past, and on whatever Nature promised them in the future, which is why the Bible depicts them as restless wanderers.