The following is taken from the first page of a 12 page eye-opening PDF which you can read here:
‘Abdullah Ibn Muhammad ad-Da’eef – rahimahullaah – said, “The Qa’diyyah are the wickedest of the Khawaarij.” Refer to Masaa’ilul-Imaam Ahmad (p.271) of Aboo Dawood.
Ibn Hajr (d.852H) said, “The Khawaarij of al-Qa’diyyah did not hold the view of waging war. Rather, they opposed the rulers in accordance to their strength, they called to their opinion and along with that they beautified revolt and held it to be good.” Refer to at-Tahdeeb(8/114) of Ibn Hajr.
Jamaal Ibn Furayhaan al-Haarithee said, “And the Qa’diyyah are, in most cases, more dangerous than the Khawaarij themselves. Since, speech, inciting hatred within the hearts and provoking the common-folk against the rulers has the most profound effect upon the souls; especially when it comes from a man who is an eloquent speaker who dupes the people with his tongue and disguises it with the Sunnah.” Refer to al-Ajwibatul-Mufeedah (p.202) of al-Haarithee.