No tyranny is more cruel than the one practiced in the shadow of the laws and under color of justice — when, so to speak, one proceeds to drown the unfortunate on the very plank by which they had saved themselves.

[Il n’y a point de plus cruelle tyrannie que celle que l’on exerce à l’ombre des lois et avec les couleurs de la justice, lorsqu’on va, pour ainsi dire, noyer des malheureux sur la planche même sur laquelle ils s’étaient sauvés.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Considerations on the Causes of the Greatness of the Romans and their Decline [Considérations sur les causes de la grandeur des Romains et de leur décadence], ch. 14 “Tiberius” (1734, 1748 ed.) [tr. Lowenthal (1965)]
    (Source)

Often mis-cited to his Spirit of Laws [The Spirit of the Laws; De l’esprit des lois] (1748).

(Source (French)). Other translations:

No tyranny can have a severer effect that that which is exercised under the appearance of laws, and with the plausible colours of justice, when the executors of cruel power would, if we may use the expression, drown the unhappy wretches on the very plank that before saved them admidst the troubled waves.
[tr. B--- (1734)]

There is no tyranny more cruel than that which is perpetrated under the color of the laws and in the name of justice -- when, so to speak, one is drawn down and drowned by means of the very plank which should have borne him up and saved his life.
[tr. Baker (1882)]

There is no tyranny more cruel than that which is exercised within the shade of the law and with the colours of justice.
[E.g.]

There is no greater tyranny than that which is perpetrated under the shield of the law and in the name of justice.
[E.g.]

There is no crueler tyranny than that which is perpetrated under the shield of the law and in the name of justice.
[E.g.]