Your statement makes errant then Paul's observation found in Romans 5:12.
If sin was in the world before Adam was created, how would you then answer God's decree concerning all that he had created? Found in Genesis 1:31.
I said that Satan was on the earth and Satan brought sin.
So, sin was found before Adam was created.
Im not sure if you realize this, but God created Satan. (Lucifer). And Satan was in heaven with God, until "iniquity was found in Him".
He was then sent OUT of Heaven, or as Jesus said....'"I beheld Satan as lightning fall from Heaven"..
Sin was IN Satan. Satan was on Earth. = Sin was on Earth.
The world, as defined as "For God so loved the World"....is not the round green planet. Its the PEOPLE....they are the "world".
So, Adam, caused a situation within all humanity that the 2nd Adam came to resolve.