What is your opinion on the serpent seed doctrine the belief that Eve had sex with satan and produced Cain?
My opinion is that the "serpent" was merely a temptation presented by the tree.
God placed the tree in the Garden to show us that our freedom is limited.
The tree allowed Adam and Eve to make right choices. It turns out it also made wrong choices possible.
God gave Adam and Eve a chance to repent but they refused. They condemned themselves. First they disobeyed God then they refused to repent.
After being cursed, we carry temptations with us. They live with us. They are our worst enemy. Eve succumbed to temptation the way we do every day, and rationalized that the fruit was good to eat.
Of course the Church teaches otherwise, and I always defer to the Church. But you asked for my opinion. And my opinion is always to place the blame on us, never on God or even on the devil.
I believe we have the powers to deny our temptations and not to succumb to them, through fervent prayer and strong faith. Evil has no power if we deny it power. God did not create evil, so evil does not 'exist' in the strictest sense.
It 'lives' in us because of what we do. We could eliminate evil in one generation just by saying "NO".