The Catman of Paris (1946)

Author Charles Regnier returns to 1896 Paris after exotic travels, having written a bestseller which the Ministry of Justice would like to ban. That very night, an official is killed in the dark streets... clawed to death! The prefect of police suspects some sort of "were-cat" but Inspector Severen thinks there's nothing supernatural about Regnier's motive. And Regnier has increasing doubts about himself when he has another hallucinatory blackout during the second killing...