To harbour ill feelings and bad thoughts about others is a major sin. Due to its inner and subtle nature, this vice is often overlooked or taken lightly. It is therefore imperative for us to constantly reflect over our inner thoughts and feelings about others and to repent for our misdeeds.
The Noble Qur'an ordains: “O Believers! Refrain from excessive negative thoughts (suspicion, assumption, aspersion). Verily some of these thoughts are sinful…” (Surah Hujurat - Verse 12)
In another verse it states : “Verily your hearing, sight and hearts will all be questioned.” (Surah Isra – Verse 36)
Additonally, we may think ill of a person on a certain matter for which he may have repented sincerely and moved on, yet in our mind we retain those ill thoughts about him on the basis of the past incident.
Hazrat Abu Hurairah t narrates that Nabi (Sallallahu Alaihi Wasallam) said, “Avoid suspicion for suspicion is the greatest lie …” (Bukhari, Muslim)
A saintly person once advised: “As far as possible have or entertain good thoughts about people. If you hear something about someone and it can be interpreted either positively or negatively, impress on yourself to choose the positive option.”
On the day of Qiyamah, good thoughts will not be accounted for, however Allah Ta'ala shall hold one accountable regarding the bad thoughts. Bad thoughts create ill feeling, enmity and jealousy for others. It is for this reason that Islam has condemned and prohibited such acts.