Surely Adam and Eve had loads of kids by this point, why would God protect a murderer?

>**Gen 4:15** ([NASB])
So the Lord said to him, “Therefore whoever kills Cain, vengeance will be taken on him sevenfold.” And the Lord appointed a sign for Cain, so that no one finding him would slay him.

[NASB]: https://www.biblegateway.com/passage/?search=Genesis%204&version=NASB