Allah tells us that He commands His servant to be just, i.e., fair and moderate, and that He encourages kindness and good treatment. As He says:

[وَإِنْ عَاقَبْتُمْ فَعَاقِبُواْ بِمِثْلِ مَا عُوقِبْتُمْ بِهِ وَلَئِن صَبَرْتُمْ لَهُوَ خَيْرٌ لِّلصَّـبِرينَ ]

(And if you punish them, then punish them with the like of that with which you were afflicted. But if you have patience with them, then it is better for those who are patient.) (16:126)

[وَجَزَآءُ سَيِّئَةٍ سَيِّئَةٌ مِّثْلُهَا فَمَنْ عَفَا وَأَصْلَحَ فَأَجْرُهُ عَلَى اللَّهِ]

(The recompense for an offense is an offense the like thereof; but whoever forgives and makes reconciliation, his reward is with Allah.) [42:40]

[وَالْجُرُوحَ قِصَاصٌ فَمَن تَصَدَّقَ بِهِ فَهُوَ كَفَّارَةٌ لَّهُ]

(and wounds equal for equal. But if anyone remits the retaliation by way of charity, it shall count as atonement for him.) [5:45] And there are other Ayat which support the institution of justice in Islam, as well as encouraging a fair and generous attitude.