Is it permissible to pray behind an imam who killed his uncle, was imprisoned for eight years, and then became an imam for the five daily prayers and the Friday sermon?

The prayer of a transgressor (fasiq) for himself is valid, as is the prayer of a congregant (ma'mum) behind him. Some of the Salaf disliked praying behind a fasiq or an innovator (mubtadi') as a form of enjoining good and forbidding evil, and boycotting him until he repents. However, if such a boycott leads to the congregant missing Jumu'ah or congregational prayer, then it is not permissible to abandon praying behind him. This is because companions like Ibn 'Umar and Anas used to pray behind Al-Hajjaj, who was an oppressive ruler. The Prophet, peace and blessings be upon him, said: "They pray for you; if they are right, it is for you, and if they err, it is for you and against them." If a killer repents and the guardians of the blood forgive him, there is no dislike in praying behind him.