The Ventriloquist, but as one of maybe a multiple villain movie.
I can just see a scene where Bats is taking the Ventriloquist away leaving the doll on a chair. All this time talking to the doll as if it were alive.
When Batman hands the ventriloquist over to the police, he decides to get the doll as evidence? Once there, he pauses because the doll has slumped over and he remembers the Ventriloquist propping the doll upright. Close up to Batman's eyes then to the doll's. Then we hear the doll (Scarface?) speak. Cut to black, The End.
I know cliched horror movie trope but done in the superhero genre. And the Ventriloquist always speaks the part of Scarface so Batman knows the voice. And Batman isn't exactly sane so...