question #1 Sahih Muslim 1065e, Book 12, Hadith 198 : What is the significance of identifying groups that deviate from mainstream Islamic teachings?

Abu Sa’id al-Khudri reported from the Messenger of Allah (ﷺ) that a group (Khawarij) would emerge from the different parties (the party of Hadrat ‘Ali and the party of Amir Mu’awiya), the group nearer the truth between the two would kill them.