Gen 3:6 And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.
Gen 3:7 And the eyes of them both were opened, and they knew that they were naked; and they sewed fig leaves together, and made themselves aprons.
Gen 3:11 And he said, Who told thee that thou wast naked? Hast thou eaten of the tree, whereof I commanded thee that thou shouldest not eat?
Gen 3:12 And the man said, The woman whom thou gavest to be with me, she gave me of the tree, and I did eat.
Gen 3:13 And the LORD God said unto the woman, What is this that thou hast done? And the woman said, The serpent beguiled me, and I did eat.
Eve did not have a child by the devil.
The enmity between the woman and the serpent is the righteous and the wicked because when they ate of the tree they had a choice between good and evil.
The sin of Adam and Eve was eating of the tree that God told them not to eat.
Cain committed a sin because of that choice not because he was a child of the devil.
Good and evil did not come about for mankind to be good and evil because Eve was impregnated by Satan having twins representing both sides.
But it came about because it was the tree of good and evil which they now had a choice to determine their fate.
So all of their offspring has a choice which some people will choose to follow the truth and some people will not.
Gen 4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
The Bible states Adam knew his wife and she conceived, and bare Cain.
So Cain came from Adam.
Also Eve said concerning Cain that she had a child from the LORD.
Both Cain and Abel came from Adam.
Gen 4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
Gen 4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
God said if Cain did right he will be accepted not that he will automatically do evil with no choice.
Cain had a choice and he chose to sin.
Gen 4:15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
Some people believe the mark on Cain was black skin so the black people are cursed which is ridiculous.
Cain went off on his own and the bloodline came from Seth so all of Cain's people would have been wiped out in the flood.
Also the different ethnic groups did not occur until after the flood when Noah's 3 sons populated the earth.
Gen 10:32 These are the families of the sons of Noah, after their generations, in their nations: and by these were the nations divided in the earth after the flood.
Gen 11:1 And the whole earth was of one language, and of one speech.
Gen 11:6 And the LORD said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do.
Gen 11:7 Go to, let us go down, and there confound their language, that they may not understand one another's speech.
Gen 11:8 So the LORD scattered them abroad from thence upon the face of all the earth: and they left off to build the city.
I do not believe the different ethnic groups came about until after the flood.