Is he should have been arrested but he was only given a slap on the wrist a hyperbole or idiom?

The phrase "he should have been arrested but he was only given a slap on the wrist" contains both an idiom and an element of hyperbole. "Slap on the wrist" is an idiom that refers to a mild punishment or reprimand, suggesting that the consequences were minimal. The implication that he "should have been arrested" adds a hyperbolic element to emphasize the perceived severity of his actions compared to the leniency of the actual outcome.