Ch. 50 page 19, panel 4 — Nami’s interior monologue: “BELLE-MÈRE!!! I can’t wait to be free…” — the grief-tone implied Bell-mère was deceased, but the chapter did not name the killer.
The payoff
Note
On-page Bell-mère death is in Ch. 78 (Bell-mère’s debut + death in same chapter — single gunshot, negative-space composition). The Ch. 80 canonization refers to the verbal-narration confirmation delivered to non-witnesses (Sanji/Yosaku/Johnny).