A question in the language of Genesis, Chapter 4 – 5

Hello Rabbi
In the dialogue between God and Cain, God says, “Therefore whoever kills Cain will be avenged sevenfold, and God will set a mark on Cain, so that no one who finds him will strike him down.”
On the one hand, God promises Cain that whoever kills him “sevenfold shall be avenged” (which simply means that he will be punished sevenfold the punishment that God gave Cain), and on the other hand, God places a mark on Cain that will prevent harm from coming to him.
Wouldn’t God’s letter 100% prevent harming Cain? If so, what is the purpose of the promise to punish those who harm Cain?
I continued to review the following verses and came across two verses that seem to be the key to understanding the matter.
In chapter 4, verse 24, Lamech says, “For Cain shall rise sevenfold, and Lemech seventy-sevenfold,” and in chapter 5, verse 24, it does not say, “And all the days of Lamech were 777 years, and he died.”
It seems to me that, in a simple way, each verse stands on its own, but on the other hand, the great similarity between them is confusing, and seemingly suggests a connection between them.
How are we supposed to understand the verses? Is there a connection between them?
Apparently, with Cain, “seven times” would be the punishment for anyone who harms Cain. With Lamech, Lamech seems to be proud of murdering a man and a child, and says “seventy-seven” and is an expression for the multiplied punishment he deserves in relation to Cain. But this does not fit with the first understanding of “seven times.”
In short, I would love to know if the rabbi has an interesting explanation that would help understand this entire passage clearly.
thanks