If anyone can tell me which lecture this benefit was extracted from, please reach out to me, barak Allahu feekum!
The Imaam, al-Haafiz Ibn Katheer (died, 774AH, rahimahullaah) stated in “Al-Bidaayah wan-Nihaayah” (10/584-585) regarding what the Khawaarij would do to the Ummah if they ever attained strength:
“If they ever gained strength, they would surely corrupt the whole of the earth, Iraq and Syria (Shaam) – they would not leave a baby, male or female, neither a man or a woman, because as far as they are concerned the people have caused corruption, a corruption that cannot be rectified except by mass killing.”
“(The Khawarij) are the worst of killers who are slain under the sky, and the best of those killed are those who were killed by them. Those (Khawarij) are the dogs of Hell. They used to be Muslim people but they became disbelievers.”
Abu Ghālibasked: “O Abu Umamah, is that your opinion?” He said: “No! Rather I heard it from the Messenger of Allah.”