How come? Sin is in us already without knowing?

Romans 5:12 (King James Version)

Death Through Adam, Life Through Christ

"¹²Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:"

The first man and woman on the earth before us, Adam and Eve, broke God's law by sinning. They didn't obey God's command and they got disconnected from God. The sin was passed from Adam and Eve through death. When Adam died, the sin passed into Adam's children. When those children died, the sin passed to their children and passed on the generations. That is how sin spread all over in the world, including us.