What is the murderer doing to inspire this? I imagine a melodramatic old school mustache twirler with a cape – the kind who ties damsels to railroad tracks and he’s making various evil expressions, warming up to do his evil business.
It makes me think of one of the opening scenes of The Imposters when Stanley Tucci and Oliver Platt are practicing acting in their bedroom. One gives the other an emotion and the other makes the face of that emotion. I imagine that this murderer in the Mousetrap is cycling through his villain faces just as those actors do in that scene.