The subject of the text is vengeance vs humbleness and forgiveness ("doing well"). Cain chose a vengeful spirit, so it says God gave it to him, because it is what he chose. Vengeance is a great deterrent to murder but it has also fueled much anger, fudes and wars. To make myself clear: I think the mark of Cain was and still is the mark of a vengeful spirit. Have you ever struggled with it? have you ever stayed your hand because of the vengeance you feared would followed on you?
I would agree the subject of the text is vengeance vs humbleness and forgiveness as the beginning of the matter . There Abel humbled himself by the grace of God that worked in him with him and did the good pleasure believing God not seen. And the father of lies a murder from the beginning began his murderous rampage.
When Abel brought the gospel in respect to believing a God not seen, to show Cain had no faith he murdered his brother as in out of sight out of mind . The same reason un converted mankind represented by the unbelieving Jews crucified the Lord of glory. Then I would think another part of the subject matter comes in, in respect to the mark, which I think is like; "mark my word I will bring it to pass". Almost as saying you are not going to get off that easy, I set the standard
Abel's blood was used to represent the gospel it is still crying out .You could say he passed it on to the Son of man who gave His Spirit in jeopardy of His own Spirit for our sin .
The punishment that was more than he could bear is in his work the fruitfulness of it has become all for nothing no future joy to look forward to. The work would be harder no rest for the wicked in that way .No Sabbath rest that could come from having a faith that can please God not seen. A restless wanderer
And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground. And
now art thou cursed from the earth, which hath opened her mouth to receive thy brother's blood from thy hand;When thou tillest the ground, it shall not henceforth yield unto thee her strength;
a fugitive and a vagabond shalt thou be in the earth. And
Cain said unto the Lord,
My punishment is greater than I can bear. Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. And the Lord said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him.Geneisis 4:10-15
Only Christ could bear the sins of those he calls to salvation. They have a future hope that gives them daily rest/sabbath as those given the privilege to mix faith in what we do see or hear, believing God not seen and not that of our own selves.