The Torah's warning to Cain — "sin crouches at the door" — becomes, in Targum Pseudo-Jonathan on (Genesis 4:7), one of the clearest statements of Jewish free will in the entire Torah.
"If thou doest thy work well, will not thy guilt be forgiven thee? But if thou doest not thy work well in this world, thy sin is retained unto the day of the great judgment, and at the doors of thy heart lieth thy sin."
The Targumist relocates the crouching sin from a literal door to "the doors of thy heart." This is internal, not external. The Yetzer Hara — the evil inclination — waits inside a person, not outside.
"Into thy hand I have delivered the power over evil passion"
Then God speaks the line that generations of Jewish ethical thought will build on: "Into thy hand have I delivered the power over evil passion, and unto thee shall be the inclination thereof, that thou mayest have authority over it, to become righteous, or to sin."
Free will, in the Targum, is not a philosophical abstraction. It is a gift God hands to Cain — and by extension, to every human being — moments before he decides whether to kill his brother. Cain is told explicitly: you can master this. The sin is not destined. The yetzer is strong but not sovereign. You rule over it, if you choose to.
Cain does not choose to. But the teaching survives him. Every person who later struggles against temptation is standing in Cain's shoes, with the same words ringing: you have the authority.