The passage in question may be interpreted simply and may also receive an elaborate explanation. For it is not the part of a righteous judge to define requital on the principle of like for like, but the originator of evil mast pay his debt with addition, if he is to be made better by punishment and render other men wiser by his example. Therefore, since it is ordained that Cain pay the penalty of his sin sevenfold, he who kills him, it is said, will discharge the sentence pronounced against him by the divine judgment. This is the sense that suggests itself to us on our first reading the passage. But readers, gifted with greater curiosity, are naturally inclined to probe into the question further.
Scripture continually assigns seven as the number of the remission of sins.
After seven years the Hebrew used to be freed from slavery. These things are types of this present life, which revolves in seven days and passes by, wherein punishments of slighter sins are inflicted, according to the loving care of our good Lord, to save us from being delivered to punishment in the age that has no end. The expression seven times is therefore introduced because of its connexion with this present world for men who love this world ought specially to be punished in the things for the sake of which they have chosen to live wicked lives. If you understand the vengeances to be for the sins committed by Cain, you will find those sins to be seven. Or if you understand them to mean the sentence passed on him by the Judge, you will not go far wrong.