There are a lot of “firsts” in the Bible, particularly in books like Genesis and Acts as they are books of beginnings. Bible scholars speak of the Law of First Mention to refer to the importance of paying close attention to the first mention of something in the Bible. The reason being, the very first time something is mentioned in the Bible we are usually given God’s original intention or pattern for it.
For example, in Genesis we find the first marriage– one man, one woman, for one life –as God’s pattern for that institution. We also find the first murder and God’s first punishment for that crime.
It’s interesting that God didn’t order Cain killed. In fact, his punishment was to be banished from society to spend his life away from the place of life God gave Adam and Eve.
It’s also interesting Cain replied: “My punishment is more than I can bear” Genesis 4:13). Putting our worst offenders out of their misery is less excruciating than decades of life in a small cell.
It’s also interesting that God marked Cain. He wasn’t allowed to blend back in but he’d forever bear stigma and separation because of his horrific deed.
It’s also interesting how God basically said to the rest of society… don’t anyone lay a finger on him: “...if anyone kills Cain, he will suffer vengeance seven times over” (Genesis 4:15).
Apparently God wants us to leave the vengeance up to Him.