They deviated, so Allaah deviated their hearts!
Mus’ab Ibn Sad (rahimahullaah) reports that the affair of the khawaarij was mentioned in the presence of his father, so he (i.e. his father) said: They are a deviated people, so Allaah deviated their hearts.’’ [Source: ‘As-Sunnah’ of Abdullah Ibn (Imaam) Ahmad 2/638]