The Hebrew Bible says God "paid regard" to Abel's offering but not to Cain's (Genesis 4:4-5). Targum Onkelos rephrases this as: "There was favor before God" for Abel's offering, but for Cain's, "there was no favor." The shift from God actively paying attention to favor existing "before" God is classic Onkelos—removing any hint that God has preferences the way humans do.
But the most theologically charged translation comes at (Genesis 4:7), where God warns Cain about sin. The Hebrew is notoriously difficult: "Sin crouches at the door... you can dominate it." Onkelos adds an entire interpretive layer: "If you do not improve your actions, your sin will be kept for the Day of Judgment. Its desire is toward you, but you can dominate it. For in the future, it will be exacted from you if you do not repent. And if you repent, there is forgiveness."
This is not translation. This is theology. Onkelos introduces the concept of a future Day of Judgment, the possibility of repentance, and the promise of forgiveness—none of which appear in the Hebrew text. For Onkelos, the Torah's terse warning to Cain already contains the entire rabbinic framework of sin, repentance, and divine mercy. He simply makes it explicit.
When Cain protests that "my sin is too great to bear" (Genesis 4:13) and says he will be "hidden from Your face," Onkelos adds a striking correction: "from before You it is impossible to be hidden." Cain thinks he can flee God's presence. Onkelos insists no one can.