Just as rust arising from iron eats away that same iron, so one’s own actions lead the transgressor to a bad destiny.

Ayasāva malaṃ samuṭṭhitaṃ taduṭṭhāya tameva khādati — as rust arising from iron eats away the iron: the impurity (mala) does not come from outside but arises from the material itself. Rust is produced by iron itself and then destroys it — a process of self-destruction from within.

Evaṃ atidhonacārinaṃ sākammāni nayanti duggatiṃ — thus one’s own actions lead the transgressor to a bad destiny: atidhonacārin is one who exceeds limits, the transgressor of discipline. Their own actions (sākamma) are the rust that corrodes them.

The rust metaphor is psychologically exact: mental impurities (greed, aversion, ignorance) are not external agents but internal products that, if unattended, consume the being from within.

As in verse 165 (the diamond crushing the gem), here self-destruction is the central theme. The impurity vagga insists that the greatest danger comes from within, not without.