As far as I know about Christian theology, yall believe that sin came into this world by Adam & Eve eating the apple.
I don't believe the fruit is named actually....it was not poisoned but represented God's sovereign authority over his creation. That is why stealing that fruit carried the death penalty. Undermining God's rightful sovereignty was a capital offense.
Taking that fruit meant stealing something that God said was his exclusive property. Nothing good would result from a knowledge of evil.
That would mean that before the apple was eaten that there was no sin in the world. So pre-apple eating, how were A&E able to decide to eat the apple?
They were given access to "every tree of the garden" except that one....so there was no hardship created by the prohibition....only a test of respect for what God said was his, and trust that he would not lie to them or withhold anything beneficial from them.
Death would result, but the first rebel said that wasn't true. (making God out to be a liar) There was no other cause of death for human beings, so pre-eating of the fruit meant that humans would have lived forever in their mortal flesh in paradise conditions that they themselves had created outside of the garden. (Genesis 1:28)
If you say that the devil/snake/Lucifer/Satan made them do it, then what is the difference in the pre-apple eating and post-apple eating humans?
No one "made" them do anything....no one can make us do anything even now. The man and his wife chose to disobey God for two entirely different reasons.
The devil targeted the woman when she was alone. Then he planted a seed of doubt and appealed to her self-interest suggesting that this good life she had, could be even better if they had a knowledge of evil because then they would be "like God". She fell for that and was deceived by him...but the crunch came when satan stood back to watch the fallout with his real target....her husband had waited a long time for his mate and now he faced losing her. The devil had succeeded in dividing Adam's loyalties and force him to choose between the love of his life and the love of his God. He chose badly and plunged the entire human race into chaos....the chaos that God had tried to spare them from ever knowing about. But once the 'genie was out of the bottle' there was no sending it back, so what transpired from there was an object lesson for all on the folly of trying to live a life independent from The Creator.
And if there was no sin in the world pre-apple eating, how could the sinful devil/snake/Lucifer/Satan even exist?
Free moral agency is the gift God gave to all his intelligent creation. The angels were free willed beings too. The devil was not created to be evil, but as Ezekiel said in his dirge to the King of Tyre, his words clearly apply to satan.
Ezekiel 28:13-15:
"You were in Eʹden, the garden of God.
You were adorned with every precious stone
—Ruby, topaz, and jasper;
chrysʹo·lite, onyx, and jade; sapphire, turquoise, and emerald;
And their settings and mountings were made of gold.
They were prepared on the day you were created.
14 I assigned you as the anointed covering cherub.
You were on the holy mountain of God, and you walked about among fiery stones.
15 You were faultless in your ways from the day you were created
Until unrighteousness was found in you."
Just as a thief or a murderer is not born to be such, neither was this cherub created to be a satan or a devil. He chose his course just as we choose ours. No one makes us do anything against our will.