The Messenger of Allah said “I have not been ordered by Allah to search the hearts of the people or to cut open their bellies” Then the Prophet looked at him while the latter (the man) was going away and said, “From the offspring of this (man) there will come out a people who will recite the Quran continuously and elegantly but it will not exceed their throats. They will then go out of the religion as an arrow goes through the games body.I think he also said, If I should be present at their time, I would kill them as the nations of Thamud were killed.” Sahih Bukhari Vol 5 #637

Abu Huraira narrated: I heard the truthful and trusted by Allah (the Prophet) saying, “The destruction of my followers will be through the hands of young men from Quraish.” Sahih Bukhari Vol 9 #180

“There shall come in another time foolish and simple minded youth. They will say the best words of creation. They will recite the Quran and it will not go past their throats. They will go out of the religion, just as the arrow goes out of the game. Then, when you establish the proof on them, then kill them. Then if you kill them, in killing them is a reward in the sight of Allah on the Day of Resurrection.”

“There will be in my Ummah differences and division. A people will speak goodly and they will do sinful acts. They will recite the Quran and it will not go past their throats. They will leave out of the religion as an arrow leaves out of the game. They will not return until a part of it apostates. They are the worst of creation and creatures. So blessings and good tidings be to the one who kills them and they kill him.”

Ali said “No, by Allah, they are still in the backs and spines of men and the wombs of women. And when are alive, they hardly leave anyone alone.” Bidayah wa Nihaya Vol 7 P295

Ibn Taymiyyah said about the Khawarij:
“And this sign which the Messenger of Allah mentioned would be the sign for the first people of the Khawaarij. But they are not the only people that are spoken of in these verses because He mentioned in other hadith that they would still keep coming until the age of the Dajjal. It is concluded by the Muslims that the Khawaarij are not the only Khawaarij mentioned in the battles with Ali in the battle of Nahraawaan.” - Majmua Fatawa Vol 28 P495-496

Ibn Taymiyyah said with regards to a helpless or weak individual of the Khawarij and the Rawafida:
“It is narrated on the authority of Ali and Umar as well as the consensus of the scholars of Islam that these two individuals should also be killed.

Some scholars argue about the individual that is not fighting. However, they all agree that to kill them in a group as a group protecting themselves with the sword is allowed. This is because fighting is more general than killing.” - Majmua Fatawa Vol 28 P476-479