I heard about a group that was named Khawarij and I'm looking for an explanation of who they were and what they believed.
I heard that they were the ones that Assassinated saidina Ali (r.a).

Ibn Maajah narrated that Ibn Abi Awfa said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The Khawaarij are the dogs of Hell.”

The Khawaarij are followers of whims and desires and innovation who have deviated from the path of Ahl as-Sunnah wa’l-Jamaa‘ah.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The Khawaarij who deviated, whom the Prophet (blessings and peace of Allah be upon him) enjoined us to fight, and whom Ameer al-Mu’mineen ‘Ali (may Allah be pleased with him), one of the Rightly-Guided Caliphs fought, and whom the leading scholars of Islam among the Sahaabah, Taabi‘een and those who came after them were unanimously agreed upon fighting, were not described as disbelievers by ‘Ali ibn Abi Taalib, Sa‘d ibn Abi Waqqaas and others among the Sahaabah; rather they regarded them as Muslims even though they fought them, and ‘Ali did not fight them until they shed blood unlawfully and raided the property of the Muslims; then he fought them in order to ward off their wrongdoing and aggression, not because they were disbelievers. Hence he did not take their womenfolk captive.



A day before Ali was killed three Kharjis met in Kufa (some say in Makkah) to hatch a conspiracy. One of the three conspirators was a certain Abdur Rahman bin Muljam. Ali was killed in the Great Mosque of Kufa

IMAliyu:
I heard about a group that was named Khawarij and I'm looking for an explanation of who they were and what they believed.
I heard that they were the ones that Assassinated saidina Ali (r.a).

Yes they were the first terrorists. It was their lack of patience and hatred of Ali R.A. that led them astray. They declared Muslims that they didn't regard and major sinners as disbelievers and killed innocent people. They were extremely rigorous and uncompromising in religious practices but very ignorant, you would even think they were better than the Sahaba because of their worshipping. Read the story of Imam Abu Hanifa and the Khawarij. For more details.