Men can be unjust, because it is in their interest to act so, and they prefer their own satisfaction to that of others. They always act with themselves in mind. No one is gratuitously wicked; there must be a determining cause, and it is always one of self-interest.

[Les hommes peuvent faire des injustices, parce qu’ils ont intérêt de les commettre et qu’ils préfèrent leur propre satisfaction à celle des autres. C’est toujours par un retour sur eux-mêmes qu’ils agissent: nul n’est mauvais gratuitement; il faut qu’il y ait une raison qui détermine, et cette raison est toujours une raison d’intérêt.]

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

Persian Letters [Lettres Persanes], Letter 84, Usbek to Rhédi (1721) [tr. Healy (1964), # 83]

Montesquieu's argues that an omnipotent God must be just, because God has no interest that cannot be satisfied through injustice.

(Source (French)). Alternate translations:

Men may commit injustice, because it is in their interest to do it, and they chuse rather to satisfy themselves and others. It is always with an eye to themselves that they act: no body is wicked gratis: he will have some reason to sway him; and that reason is always a reason of interest.
[tr. Ozell (1760 ed.)]

Men may do injustice, because it is in their interest to commit it, and because they prefer their own private satisfaction to that of others. It is always with a view to themselves that they act: nobody is wicked for nothing: he must have some reason that determines himl and this reason is always a reason of interest.
[tr. Floyd (1762), # 83]

Men act unjustly, because it is their interest to do so, and because they prefer their own satisfaction to that of others. They act always to secure some advantage to themselves: no one is a villain gratis; there is always a determining motive, and that motive is always an interested one.
[tr. Davidson (1891)]

Men act unjustly, because it is their interest to do so, and they prefer their own satisfaction to that of others. In acting they always have in view the effect their action will have on themselves: no one is bad for nothing; every one must have a determining motive, and that motive is self-interest.
[tr. Betts (1897)]

Men can commit injustices, because it is in their interest to do so, and they would rather satisfy themselves than others. It is always through thinking of themselves that they act unjustly; no one is gratuitously bad, there must be a reason which determines the act, and that reason is invariably one of self-interest.
[tr. Mauldon (2008)]

Men are capable of injustice, because their self-interest leads them toward it, and because they prefer their own satisfaction to that of others. Everything always revolves around themselves. No evil is ever done gratuitously, for there is always a reason behind it, and that reason is always one of self-interest.
[tr. MacKenzie (2014), # 83]