“There can be as many wrong reasons to do the right thing as there are stars in the sky. There might even be more than one legitimate right reason. But there is never a right reason to do the wrong thing. Not ever.”

From 1980s onwards, Cosmography (1992)

Source: The Phantom Tollbooth

Variant: The last act is the greatest treason. To do the right deed for the wrong reason.
Source: Murder in the Cathedral

From 1980s onwards, Cosmography (1992)
Context: Doing the right things for the wrong reasons is typical of humanity. Precession — not conscious planning — provides a productive outcome for misguided political and military campaigns. Nature's long-term design intervenes to circumvent the shortsightedness of human individuals, corporations, and nations competing for a share of the economic pie. Fundamentally, political economists misassume an inadequacy of life support to exist on our planet. Humanity therefore competes militarily to see which political system... is fittest to survive. In slavish observance of this misassumption, humans devote their most costly efforts and resources to "killingry" — a vast arsenal of weapons skillfully designed to kill ever more people at ever-greater distances in ever-shorter periods of time while employing ever-fewer pounds of material, ergs of energy, and seconds of time per killing.

Is Divorce Wrong? (1889)
Context: Marriages are made by men and women; not by society; not by the state; not by the church; not by supernatural beings. By this time we should know that nothing is moral that does not tend to the well-being of sentient beings; that nothing is virtuous the result of which is not good. We know now, if we know anything, that all the reasons for doing right, and all the reasons against doing wrong, are here in this world.

Source: The Sweet Far Thing

Presumably a paraphrase of "A peculiar virtue in wildlife ethics is that the hunter ordinarily has no gallery to applaud or disapprove of his conduct" or of "Hunting for sport is an improvement ..." above.
Unlikely to be by Leopold, who knew that ethics involves not only doing the right thing, but also determining the right thing in the face of competing desirable criteria.
Misattributed

Hansard http://www.publications.parliament.uk/pa/cm200506/cmhansrd/vo051109/debtext/51109-03.htm#51109-03_spmin10, House of Commons, 6th series, vol. 439, col. 302.
9 November 2005, responding to Charles Kennedy in the House of Commons during Prime Minister's Questions. Blair was referring to the likely defeat in Parliament of additional powers to detain terror suspects without charge, which happened later that day.
2000s

Source: The Great Crash, 1929 (1954 and 1997 https://openlibrary.org/books/OL25728842M/The_Great_Crash_1929), Chapter V, The Twilight of Illusion, Section VII, p. 85