Bism Illah wa as salaamu alaykum.
Rancour, or rancor if you prefer the US English spelling is defined as bitterness or hatred towards someone. It is an extremely strong emotion and one I have both considered in my life and discussed with friends. The discussion usually goes along the lines of it being a bad feeling and something we should avoid or fight in ourselves. It is sometimes connected with a feeling towards someone who we believe we should be forgiving and maintaining the ties of sisterhood with or should be nicer to and allow more of our time.
Firstly, let me clearly state that I have no bitterness or rancor in my heart for people who have wronged me. Truly, they have not harmed me, they have harmed themselves and their account with Allah, subhaana wa taala. He is The Fair, The Just...I have not a single worry about that. May Allah guide them and me to a better place - ameen.
However, I do have bitter hatred for shaytaan, his traps and plots, his long-term machinations, and his friends, allies, and helpers. I have no doubt that it comes purely from hating what Allah, azza wa jal, hates. Therefore, I cannot consider it wrong to have this feeling if I am truly a believer, submitting my whole self to my Creator.
The problem is, how on earth do I deal with it? How can I find an Islamic solution to dealing with people whose behavior is abhorrent to me and for whom I feel rancor? Moreover, how can I tone down my feelings and reactions to behaviors that shaytaan loves? I may not have rancor for the person, but their behavior evokes strong feelings of abhorrence.
After much struggling, I have found that avoidance and is the best policy as far as possible. After all, we are on the deen our our companions, so one will not draw ever closer to those who don't love what Allah Loves. I just leave it to Allah and get on with my life. But when I cannot simply extricate myself from those people or situations, dealing decently, having manners, patience, and diplomacy is important. I cannot hide my feelings well, so it is important that, at least, I hold my tongue. If I cannot speak in a polite and gentle way, then shut up.
That's where I am right now. Every now and then I end up saying something, not in the best of ways but not wrongly or rudely, that leaves me feeling I have a long way to go on this subject. May Allah help and guide me and all who struggle with this problem - ameen.
One thing that comforts me, lets me know that I'm not alone in my feelings or struggles is the knowledge that our beloved Prophet, sal Allahu alayhi wa sallam, used to make dua' to Allah to remove rancor from his heart. What a blessing and a mercy our Prophet, sal Allahu alayhi wa sallam, is to us all..wa al hamdu l'Illahi Rabbil aal Ameen!