Toshihiko Izutsu writes on the khawarij sect called Muhakkimah:
Those Muhakkimah used to go out with their swords to the market places. And when the innocent people gathered together without being aware of it, they suddenly cried out ‘La hukm illa lillah!’ and lifted up their swords against anybody they happened to overtake, and they went on killing people until they themselves were killed. ….The people used to live in constant fear of them; it caused a terrible commotion. But fortunately not even one of them remains now on earth.
[“The concept of belief in Islamic theology” (p.7) by Toshihiko Izutsu referencing his source as “al Tanbih wa-al Radd Ala Ahl al Ahwa wa al bidah” of Al-Malati.]
Sayyid Abdul Qadir al-Gilani al-Hanbali in his book “al-Ghunya li-Talibi Tariq al-Haqq”, describes the traits of a Khawarij sect named “Azariqa”:
The Azariqa also consider it permissible to kill young children, meaning the offspring of those who attribute partners to Allah [awlad al-mushrikin].
and that of another Khawarij sect named “Bida’iyya”:
They are in agreement with the Azariqa on the permissibility of taking women captives from among the unbelievers [kuffar], and of killing their infant children inadvertently, on the strength of [their interpretation of] the words of Allah (Exalted is He): [And Noah said: “My Lord,] do not leave upon the earth even one of the unbelievers.” (71:26)