وَلَا تَحْسَبَنَّ ٱللَّهَ غَٰفِلًا عَمَّا يَعْمَلُ ٱلظَّٰلِمُونَ ۚ إِنَّمَا يُؤَخِّرُهُمْ لِيَوْمٍۢ تَشْخَصُ فِيهِ ٱلْأَبْصَٰرُ ﴿٤٢﴾
你绝不要以为真主忽视不义者的行为，他只对他们缓刑到瞪眼的日子，
Haram Speech & character
Is oppression (zulm) haram?
Short answer
Yes. In a hadith qudsi Allah says that He has made oppression unlawful for Himself and unlawful among people, so do not oppress one another. The Quran warns that Allah is not unaware of what the wrongdoers do (14:42).
Evidence
Quran
إِنَّمَا ٱلسَّبِيلُ عَلَى ٱلَّذِينَ يَظْلِمُونَ ٱلنَّاسَ وَيَبْغُونَ فِى ٱلْأَرْضِ بِغَيْرِ ٱلْحَقِّ ۚ أُو۟لَٰٓئِكَ لَهُمْ عَذَابٌ أَلِيمٌۭ ﴿٤٢﴾
应受责备的，是欺侮他人，并且在地方上蛮横无理者；这些人将受痛苦的刑罚。
Hadith
Sahih Muslim 2577d
Abu Dharr reported Allah's Messenger (ﷺ) as saying that he reported it from his Lord, the Exalted and Glorious:Verily I have made oppression unlawful for Me and for My servants too, so do not commit oppression. The rest of the hadith is the same
Arabic text (with chain of narrators)
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، وَمُحَمَّدُ بْنُ الْمُثَنَّى، كِلاَهُمَا عَنْ عَبْدِ الصَّمَدِ بْنِ عَبْدِ، الْوَارِثِ حَدَّثَنَا هَمَّامٌ، حَدَّثَنَا قَتَادَةُ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَبِي أَسْمَاءَ، عَنْ أَبِي ذَرٍّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِيمَا يَرْوِي عَنْ رَبِّهِ تَبَارَكَ وَتَعَالَى " إِنِّي حَرَّمْتُ عَلَى نَفْسِي الظُّلْمَ وَعَلَى عِبَادِي فَلاَ تَظَالَمُوا " . وَسَاقَ الْحَدِيثَ بِنَحْوِهِ وَحَدِيثُ أَبِي إِدْرِيسَ الَّذِي ذَكَرْنَاهُ أَتَمُّ مِنْ هَذَا .
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Questions about halal and haram
Is oppression (zulm) haram?
Yes. In a hadith qudsi Allah says that He has made oppression unlawful for Himself and unlawful among people, so do not oppress one another. The Quran warns that Allah is not unaware of what the wrongdoers do (14:42).