The Proofs for the Justice of Allāh, the Mighty and High, are many. We will mention a few here:

First Proof:
Every human being, even though he may not be religious, perceives through his nature the goodness of justice and the evil of oppression. In fact, even if the oppressor himself was described as an oppressor, he would feel hurt and would dislike it. On the contrary, if he was described as just, he would feel delighted and pleased.

If a criminal and oppressor, who wastes all his aspirations following his desires and anger, was taken to court and the judge rules for him falsely due to bribe or fear of the criminal’s power, then the criminal would feel pleased, but his logic and instinct would rule against the judgment. Hence, he will believe the judge to be lowly.

On the contrary, if the judge does not take the bribe or the criminal’s position into any consideration and rules with justice, then the oppressor would be angry at him. However, his instinct will look at the judge and his ruling with respect and admiration. If this is the case with a human being, then how is it possible for Allāh, Glory be Him, to be an oppressor in His sovereignty and rule? It is He who made human nature to accept justice as good and injustice or oppression as evil. He intends for the human being to adapt to justice and refrain from the dirt of oppression. He, the Exalted, has commanded through His Word: Surely Allah enjoins the doing of justice and the doing of good,1and Say: My Lord has enjoined justice,2and O Dāwūd ! Surely We have made you a ruler in the land; so judge between men with justice and do not follow desire.3

Second Proof:
Certainly, oppression occurs due to one of three reasons and all of them are impossible to be related to Allāh: either due to ignorance of the evil; or due to incapability in reaching the goal except through committing oppression; or due to nonsense and vanity. Allāh is free from ignorance,....

reference:
1 Qur’ān: 16: 90.
2 Ibid: 7: 29.
3 Ibid: 38: 26.


......incapability and nonsense. Thus, His Knowledge of everything, His Power over everything and His Wisdom that reaches everything requires that He be Just and free from any kind of oppression and evil.

Third Proof:
Oppression is a deficiency. If Allāh, the Exalted, was an oppressor then it would necessitate that He was composed of both deficiency and perfection, and of existence and nonexistence. This is the worst of the types of compositions, as mentioned earlier. In addition to that, the composed of both perfection and deficiency is in need and limited. Neediness and limitation are attributes of creation and not the Creator. In conclusion, Allah, Blessed and Exalted be He, is Just in creating the universe: Allah bears witness that there is no god but He, and (so do) the angels and those possessed of knowledge, maintaining His creation with justice; there is no god but He, the Mighty, the Wise.1 He is Just in His laws and regulations: Certainly We sent Our messengers with clear arguments, and sent down with them the Book and the balance that men may conduct themselves with equity.2 And He is Just in the accountability of His servants on the Day of Judgment: And the matter shall be decided between them with justice and they shall not be dealt with unjustly.3 A man asked Imām Jaᶜfar al-Sadiq (A): Verily, Divine Unity and Divine Justice are the foundations of faith. Since His Knowledge is so great, it is not possible for even a wise man to understand it all. So, teach me something that is easy to understand and easy to memorise. He (A) answered: As for Divine Unity, do not describe your Lord the way you describe yourself. As for Divine Justice, do not apply to your Lord any of the shortcomings that you may apply to yourself.4 He (A) said to Hishām ibn al-Ḥakam: Should I not give you a sentence concerning Divine Justice and Divine Unity? He said, “Of course. May I be your ransom!” He replied: Justice is not to blame Him and Divine Unity is not to imagine Him.5 The Commander of the Faithful (A) said: Whatever you repent for from Allāh, the Exalted, is from you, and whatever you praise Allāh, the Exalted, for is from Him.6

Reference:
1 Qur’ān: 3: 18.
2 Ibid: 57: 25.
3 Ibid: 10: 54.
4 Shaykh Ṣadūq, Divine Unity: pp 96, ch. 5, ḥadīth 1.
5 Biḥār al-Anwār: vol. 5, pp 58.
6 Al-Ṭarā’if: 329.