Is the understanding that "the Khawarij are the dogs of the People of Hellfire" correct in that it means they are from the Majus (Zoroastrians), and am I sinful for giving a fatwa without knowledge, and what is the expiation for that, and what is the correct meaning of the hadith?

The Kharijites were the first innovation (bid'ah) to appear in Islam. Scholars differed concerning whether they should be declared disbelievers (takfir), and the majority of scholars hold that they are not disbelievers. Ibn Majah and Ahmad narrated from Ibn Abi Awfa, and At-Tirmidhi from Abu Umamah, that the Messenger of Allah, peace and blessings be upon him, said: "The Kharijites are the dogs of Hellfire." The meaning of this could be that they will be in Hellfire in the form of dogs, or that they will howl therein like dogs, or that they will be the most despicable of its inhabitants. As for the interpretation that they are the dogs of Hellfire because they are the Zoroastrians (Majus) of this Ummah, this interpretation is incorrect. The Zoroastrians of the Ummah are the Qadariyyah, who deny that Allah Almighty predetermined the destinies of creation.