I can see how you think that.
Let's help here. Maybe dr Cadry needs to assist us...
Capital punishment was a sanction whereby God dispenses man with the authority to put another man to death to punish that man for a crime.
Your argument shows a pre conceived philosophical prejudice -
God destroying the earth with a flood is not a decree of capital punishment and actually the opposite is true because God places a K on cain's forehead to prevent some samoan from placing a rock pile on him.
Now once you can digest this milk we shall go for a main course my son
the dear dr Cadry is a few bricks short...
What was Cain's concern in the first place? "...and it shall come to pass, that every one that findeth me shall slay me" (Genesis 4:14 KJV). God didn't kill him... why was Cain concerned about everyone else? Why would anyone want to place a rock pile on him? Btw, the flood sorta took out Cain's descendants, except for one, Ham's wife, grandmother of Nimrod, king of Babel (HINT). Noah, & Co. survived. That was God's decree that he later inspired Moses with finer distinction... unless you think Moses was hearing strange voices coming up with those Laws?
Why was Cain afraid, son? I got the 'everyone would kill him.' Why would anyone want to kill Cain?