If the world never knew death before Cain slew Able was it really murder for Cain to kill him? No one knew what the outcome of being hit in the head with a rock would be so in reality Cain was innocent of the charge.
Another story that they did not think all the way through.
It is interesting that in the original Hebrew Cain and Abel are not names, they are titles Able means The Breath and Cain means the spear.
So in effect the children of the Adam (the red earth) and Eve (the giver of life) are life and death .
I suppose there is some clumsy symbolism here.
You could say that god had explained about death to Adam and Eve when they ate the fruit, or that the knowledge came with the fruit. But it is perhaps best not to take the story so literally, the killing is only representative of the entry of sin into the world, and makes the metaphorical point that after leaving the garden of Eden people would have to compete for resources. Still twaddle though.