Harbouring malice, ill feelings and bad thoughts about another Muslim is a major sin.
To disagree, differ and, at times, experience discord with another person is normal and expected. However, a Muslim is prohibited from allowing these issues to cause him to sever his relationship with the next person.
Irrespective of the level of conflict or disagreement that may exist between two parties, it is prohibited for them to hate, spite or grudge each other or harbour ill feelings and hatred. Such an approach will be detrimental spiritually and can also affect one's physical health.
Harbouring enmity leads to distress, anxiety and restlessness. The one who holds grudges and gives vent to his anger tortures himself with the fire of jealousy and hatred. Such a person loses respect and self-esteem. The evil effect of these actions engulf his heart, causing him to become miserable and depressed.
In upholding relationships, the Qur'an advises us to take a higher moral ground,
"The good deed and the evil deed cannot be equal. Repel (the evil) with (an approach which is better) then verily (the one) with whom there was enmity, will become as though he was a close friend." (Fussilat: 34)
Thus Allah orders the faithful believers to be patient and tolerant at the time of anger, and to take to pardon those who treat you badly.
Hazrat Anas (R) narrates that Rasulullah (Sallallahu Alaihi Wasallam) said to me, 'O My Beloved son! If you are able to pass the morning and the evening in such a condition where there are no ill-feelings in your heart for anyone, then do so.'
Then Rasulullah (Sallallahu Alaihi Wasallam) said, 'O My Beloved son! That is from my Sunnah. Whoever loves my Sunnah loves me. Whoever loves me will be with me in Jannah.' (Tirmizi)
We should develop ourselves to be tolerant with others, and overlook the weaknesses of others by displaying good character.