TT #30 · Parashas Vayeishev · Passuk 38:10
Parashas Vayeishev · Passuk 38:10
And what he did was evil in the eyes of Hashem; and He slew him also.
Torah Temimah points to Niddah 13a (R’ Yochanan).
Sefaria — Niddah 13a (R’ Yochanan)
R’ Yochanan said: Whoever emits seed in vain is liable to death, as it is stated: “And what he did was evil in the eyes of Hashem; and He slew him also.”
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The death of Onan is read as the paradigm for the severity of this act.
Emitting seed in vain is so severe that the Torah records death as its consequence in Onan’s case.