Cain’s answer says so many things. First, he looks more to himself than to God with whom he speaks. He sees the boulder that his conscience – from then on – will have to endure and, according to its narrow parameters, such a big fault can not be forgiven. In Hebrew guilt is awon and has two meanings: a. iniquity, or something wrong according to the parameters of the law; 2. punishment for such iniquity. Probably Cain guessed the blame in the second sense. He thought of the irreparable and unbearable consequences of what he had done. And he was right. Killing a man leads to a grave and unbearable inner weight, regardless of any commandment.
This song is well before the promulgation of the Mosaic Law in the Book of Exodus. Chapters 1-11 discuss humanity in general and the consequences of the growing moral depravity that will eventually lead to deluge. The story of Genesis 4 lies within this tale. Killing is both a sin (Genesis 4:7) that is a real evil and a guilt, something that weighs on consciousness.
It is not the sense of guilt, that is, of the infamous inner condition of those who don’t do anything without feeling guilty, without thinking of having so annoyed someone. We could say that this is the pathological fault. Here it is described the true blame, the one committed in perfect autonomy of judgment and after God – or the Cain’s consciousness – had spoken clearly. Certainly, this is an act dictated by Cain’s envy, but no state of inner discomfort will ever justify the real sin-murder.
Genesis 3 we never talks of sin. Genesis 4 speaks about the real sin, the real evil: the violence that pushes to suppress other lives. Something that, if consciousness has not yet been silenced, screams inside the man and forces him to bring unbearable weight. Will he find the pardon he asks?