For sinas chinom, look up the Rashbam, Ramban on לא תשנא את אחיך בלבבך do not keep your hatred bottled up in your heart but admonish him about it. See also the Yad Yosef on the Ein Yaakov Yoma 9. According to this there are two causes 1) not admonishing, telling him why you hate him 2) hating for an inexcusable reason where your reason for hatred is not an acceptable reason so you act like friend and keep the hatred fester in your heart.
