This is a Tabboo of the Bible.By definition it cannot be understood,because the knowledge is still to be learned.Cain represents the left path and Abel the right path.We would have a good idea that this is the case,but killing in this way is taboo.Is the knowldge of good and evil is this?
God said to Cain that sin lies at the door.Having slain Abel and moved from Cainan to Nod,he marries and then the city of Enoch is built,signifying a different path The inner life of a city.From the open face of God to the hidden face of god.It is still god, so their are two truths,not just one.The meaning being that the path of good and evil are represented by life and death, light and dark, right and left.This would be the clear cut meaning in Genesis.It seems that this is meant to be.Physical mortality.Not a mistake.
The meaning is very much in the eye of the beholder to put their own interpretation onto what is good and what is evil.Clearly the concience is the arbitrator and decider.So it is with the source of good and evil, original sin.Concience decides what is good and what is evil.This chapter is insinuating that Cain is not correct,not only to kill,but to sin.Original sin is the source meaning.So he goes the other path.Mortality is tacitly supported,but it does not admit it.It therefore redirects onto the concience of Cain to resolve this.Originla sin is redirected onto the concience of Adam and Eve to resolve.It tacitly approves, therefore of sin and death thereof.It dictates both paths,that of good and of evil.But it does not define what the knowledges of good or evil are.It is not outlawing evil or making it taboo,as we are conditioned to believe.It supports both paths equally.But it does not define what is good or what is evil,this is left to us to find out.It leaves it to our concience to decide,no one else.Our mind belongs to us and our concience.We can remain blind and not accept this if we wish.The reason mortality exists is that we need to return into time to learn these truths.(through reincarnations).
There is a great verse in the Bahgavad Gita which says nature compels,so you have to take the path willingly or unwillingly.
It is the same here.It is not only Cain that nature compels to kill Abel,it also compels Abel to kill Cain.But the solution is not arrived at physically.It is solved at a higher level.This is the same for original sin.Nature also compels this.The solution to the problem of resulting death and rebirth is arrived at at a higher level.This needs the knowledge,through experience..