The sudden death of Judah's second son illustrates a direct and piercing divine response to selfishness and moral corruption. It establishes a tragic parallel to the fate of his older brother, Er, showing a tight connection between sin and ultimate punishment.
The primary approach among commentators is that both brothers died due to the exact same offense of wasting their seed [רשב״ם, רד״ק, רבנו בחיי]. However, their underlying motives were entirely different. Onan acted out of resentment, knowing that any offspring produced would not carry his own name but would instead be attributed to his deceased brother. In contrast, Er acted out of a selfish desire to prevent his wife, Tamar, from conceiving, simply to preserve her physical beauty [רשב״ם, רבנו בחיי].
The physical act of wasting seed was severe enough on its own to warrant the death penalty, completely separate from the refusal to provide an heir for a brother. Operating on a principle of measure for measure, the very act intended to prevent new life resulted in Onan losing his own life and his chance to ever have children [העמק דבר]. While Er's specific offense remained hidden in the broader narrative, the exact nature of Onan's actions is explicitly detailed [ביאור שטיינזלץ].
The profound wickedness of this act links Onan's behavior directly to the sins of the generation of the Flood and the people of Sodom, who were similarly condemned for acts of corruption [רבנו בחיי]. Furthermore, indulging in sinful thoughts and sinking into lust defiles a person's soul. Such behavior coarsens human feelings, effectively preventing a person from remaining in God's presence or holding onto pure thoughts [תורה תמימה].
The severe punishment of the two brothers raises a question, as tradition indicates they died at the very young age of eight [ביאור שטיינזלץ]. This is surprising, given the principle that the heavenly court typically does not punish a person before the age of twenty. The resolution is that this age limit applies only to the collective punishment of an entire nation. When an individual is judged, the punishment relies on their mental maturity and awareness rather than their chronological age. Even a young child can be cunning and fully aware of their actions. Because God examines the heart and mind, the brothers were held fully accountable for their conscious choice to do evil [ריב״א, הדר זקנים].
The hidden nature of the brothers' sins had a fateful impact on Tamar. Because Judah was completely unaware of his sons' corrupt actions, he mistakenly concluded that Tamar was a cursed woman whose bad fortune caused the deaths of her husbands. Had he known that each son died strictly because of his own personal sin, Judah would never have harbored any fear of her [משכיל לדוד]. Finally, an ancient tradition suggests a distinction in how the brothers were remembered. While standard mourning practices were not observed for the firstborn, Er, the very name Onan stems from a word for mourning, hinting that his father was destined to mourn for him [תורה תמימה].