An unpunished offence is bound to be repeated

When I was still very young I often used to hear from my elders this

Spare the rod and spoil the child

Said to mean that if you do not punish a child when they do something wrong, they will not learn what is right

[Cambridge Dictionary]

Another idiom you might like is

give him enough rope and he'll hang himself

If one gives someone enough freedom of action, they may destroy themselves by foolish actions.

[Wiktionary]

Wolfgang Mieder, A Dictionary of American Proverbs (1992) offers a saying that seems to be at least somewhat on point:

Pardoning the bad is injuring the good.

That is, when a person who does wrong pays no price for the misdeed, the innocent will pay for it instead—presumably either by not receiving restitution for the original wrong done to them or by being exposed to future misdeeds committed by the same malefactor or by being harmed by others who are emboldened by the malefactor's example of unpunished misconduct.

Mieder notes that this proverb goes back to at least 1732 in English. It appears in Thomas Fuller Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings, Ancient and Modern, Foreign and British (1732), although it is not particularly common in English today, as far as I know.