Ibn Majah

Commentary:

➊
Regarding the authentication of this narration, our esteemed researcher writes that this narration is weak in its chain of transmission; however, the hadith in Sahih Bukhari (7562) suffices for it. In addition, Shaykh al-Albani rahimahullah has also declared this narration to be authentic. Therefore, it is understood that despite being weak in its chain, this narration is still valid as evidence.

➋
Shaving the head being a sign of the Khawarij does not mean that whoever shaves his head is a Khariji; rather, it only means that this habit will be found among them. Otherwise, Ali ibn Abi Talib radi Allahu anhu himself always shaved his head, while the Khawarij were his staunch enemies.
This is just like, for example, if one of the signs of Sikhs is mentioned as keeping a beard, then people start calling everyone with a full beard a Sikh.
It is obvious that saying or thinking such a thing is nothing but ignorance.
Similarly, some people of innovation (ahl al-bid‘ah) try to portray the Ahl al-Hadith as Khawarij for performing long prayers according to the Sunnah, reciting the Qur’an with zeal and devotion, and for shaving their heads instead of keeping fashionable hair, which is completely contrary to the facts, an expression of ignorance, and a denial of the importance and virtue of acting upon the Sunnah and true Islam.
A‘adhana Allahu minha (May Allah protect us from it).

➌
“Kill them” means to fight them so that their tribulation (fitnah) may be eliminated.