Where did sin come from?

According to scripture the origin of sin is found in free will.[1] God created moral beings, both angelic and human, and gave us the power of free choice. Both angels and humans miss-used that choice and as a result sin entered into God’s perfect creation.
Sometime between Genesis 1:1 and Genesis 1:2 an angel that is now called Satan chose to sin. But who was he? In Ezekiel 28:11-19 we get a glimpse of who he was.
First off how do we know that this is speaking of Satan? We understand this because of the statements in Verses 12 and 13 "You had the seal of perfection, Full of wisdom and perfect in beauty. You were in Eden, the garden of God; Every precious stone was your covering”. When we consider the underlined text we can conclude that this was no man. Verse 13 also speaks of the “day you were created” not on the day you were born as we would expect if this was speaking of a man. Also in verse 15 we read that “unrighteousness was found in you”, not that you sinned. This is clearly speaking of the first ever sin. Also in verse 15 we see “You were the anointed cherub who covers”, the word covers can also be rendered shields implying that this being was a guardian of some kind. So the total picture that we get here is of a perfect sinless being that guards who fell from that sinless state.
So what happened? Verse 6 tells us that he sinned, but not what he did. In order to see that we must look to Isaiah 14:12-15.
Isa 14:12-15 “How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations! (13) "But you said in your heart, 'I will ascend to heaven; I will raise my throne above the stars of God, And I will sit on the mount of assembly In the recesses of the north. (14) 'I will ascend above the heights of the clouds; I will make myself like the Most High.' (15) "Nevertheless you will be thrust down to Sheol, To the recesses of the pit.
Here we see the five “I wills” of Satan. Here we see Satan make statements of what he will do using the language of God when He makes an unconditional covenant. (See Genesis 12:2) Put simply this was the sin of pride; he had a desire for a place and a position that was not for him. In the Isaiah passage we get the “what” in order to get the “why” we must return to the Ezekiel passage.
In Ezekiel 28:16-18 we can get a since of why Satan sinned. "By the abundance of your trade You were internally filled with violence”, here we can infer that he lusted after wealth and power and was willing to do anything to acquire them. “Your heart was lifted up because of your beauty” We know by the description of him from earlier that Satan was a beautiful creature and from this we see that he knew it and became vain. “You corrupted your wisdom by reason of your splendor” because he was wise he substituted his wisdom for God’s wisdom.
In Romans 12:3 we read “For through the grace given to me I say to everyone among you not to think more highly of himself than he ought to think; but to think so as to have sound judgment, as God has allotted to each a measure of faith.”
The Romans passage sheds light on Satan’s problem. He became unsatisfied with his position and thought that he was something that he was not; he thought that he was God. Because of this he sought to take a place that was not his to occupy.
Ok so we have seen the angelic fall, what about humans? For that we need to turn to Genesis.
In Genesis 1:27-28 we read of God’s creation of man. Take note that man is created in the image of God, and that God intended the male female relationship even before Eve was created.
In Genesis 2:15-17 we read that God placed man in a specific location, made provision for him and placed a limit on his freedom. With that limit came a consequence. We read in verse 17 “for in the day that you eat from it you will surely die." Literally dying you will die. Or in other words dying spiritually you will die physically.
In Genesis 3:1-7 we see the temptation of Eve and her fall. Satan questioned Eve, "Indeed, has God said”, and Eve ether had not been taught correctly or had misunderstood the teaching as can be seen with her reply. “God has said, 'You shall not eat from it or touch it, or you will die.” Emphases added. Misunderstood teaching opened the door for Satan. “You surely will not die!” and “you will be like God”. Then Eve substituted her own wisdom for God’s. She saw, delighted and desired, and then she ate.
“and she gave also to her husband with her, and he ate.” A person in sin will seek to bring others into what they are doing.
“Then the eyes of both of them were opened” note that the eyes were not opened until Adam sinned. Eve was deceived, Adam was not, and he sinned willfully. In Genesis 3:8-21 we read of God seeking fallen man and providing an opportunity for confession, they did not confess, they provided excuses for what they had done and God applied discipline to them. God also provided the first hope of salvation in verse 15 with the seed of the woman promise, and God’s provision for covering with animal skin verse 21.

[1] Dr. Norman Geisler, Systematic Theology, Volume Three p. 82.