The truth is this.
Was Sin apart of GODS original purpose? No. He gave Adam and Eve perfect human life. They could not grow old, they could not get sick, they could not die, thus they were perfect humans who were obedient to GOD at first so sin did not exist. Genesis 1:28,2:8,9,15
GOD intended for humans to live forever on earth. Psalm 37:29 Revelation 21:3,4
God did not create the earth for nothing. Isaiah 45:18
GOD did create the once good angel who is named satan the devil and infact the bible does not give glory to ones original name. Satan means Resister and Devil means Slanderer . Satan was a perfect angel just like all the other myriads of angels. GOD gave angels free will just like humans. When satan was looking down at Humans he must of thought that they should worship him or they could rule themselves and did not need GODS help. Thus calling GOD a liar. Genesis 3:1-5 This once good angel allowed a wrongful desire to build inside him and when he acted on it, being powerful he made it look as if a snake was talking to Eve and deceived her into eating from the forbidden tree and Adam followed suit. They willing disobeyed GOD and thus sin entered into the world. In turn The once good angel became satan the devil.
Romans 5:12 this is why we grow old, get, sick and die,
To illustrate, how does an honest person become a thief? He was not born a theif. But he desires, or wants something that does not belong to him. He KEEPS THINKINH ABOUT IT, and his wrong desire becomes stronger. Then, when he gets the chance, he steals it. He has turned himself into a thief.
Read James 1:13-15