بسم الله الرحمن الرحيم
(Fatwa: 715/582/B=1434)
One of the characteristics of imamah is that the imam should be pious. A pious person is one who avoids major sins. If any imam is committing any major sin then he is impious. It is forbidden to make an impious person imam.
لا يؤم فاجر مومناً (ابن ماجه)
Hence salah shall be makrooh tahrimi behind such Imam.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
