Why did Ali (may Allah be pleased with him) fight the Khawarij?

The Khawarij are among the worst of the people of innovation and corruption. The Prophet, peace and blessings be upon him, condemned them and commanded fighting them, as they recite the Qur’an but it does not go beyond their collarbones, and they abandon Islam like an arrow piercing its target, killing the people of Islam and leaving the idolaters alone. Ali ibn Abi Talib, may Allah be pleased with him, fought them after they shed forbidden blood and raided the wealth of Muslims. He did not declare them disbelievers but rather considered them Muslims; he fought them to repel their injustice, not for their disbelief. Among the signs of their deviance is their killing of Abdullah ibn Khabbab and their disemboweling of his pregnant wife without right.