Is Abdullah ibn al-Khuwaysirah al-Tamimi, by his saying to the Prophet, peace and blessings be upon him, "Be just," considered an apostate or a hypocrite? And what are the sayings of the scholars regarding that?

Dhul-Khuwaisirah, the leader of the Khawarij, was a hypocrite. This is evident in the Quran where Allah says: (And among them are those who criticize you concerning the charities) [Quran 9:58], and in his statement to the Prophet, peace and blessings be upon him: "Be just and fear Allah," thereby attributing injustice and lack of piety to him. The Prophet, peace and blessings be upon him, did not kill him because he outwardly professed Islam. He pardoned him to win over hearts and so that people would not say that Muhammad kills his companions. Another indication of his hypocrisy is 'Umar ibn al-Khattab's, may Allah be pleased with him, calling him a hypocrite, and the Prophet's, peace and blessings be upon him, not rebuking 'Umar for that.