AN ACT OF INJUSTICE
Kindness is only kindness when it does not involve doing a forbidden act or neglecting to do an obligatory one.
If a kindness involves abandoning justice, it becomes oppression. This is why showing favouritism towards some children above others and to some wives above others is forbidden even though it might be kindness towards the one being preferred and favoured, because it is an act of injustice.
Book: Lessons Learnt From The Story Of Yūsuf (عليه السلام)
By Shaykh 'Abd ur-Rahmān ibn Nāsir as-Sa'dī
Translated by Abū 'Abdillāh Nūr ad-Dīn
P. 122, 123