Praying behind a sinner, an innovator, an oppressive leader or somebody who is hated by those whom he leads.
Bukhārī said in his Ṣaḥīḥ, in the chapter concerning those who have been sent a trial/tribulation or an innovator that Al Ḥasan said, “Pray behind him and his innovation is his problem” [Fatḥ Al Bārī 2/158]