Movie: Girl in the mist (La ragazza nella nebbia)

Usually the killer commits twenty errors. But sees only a third of them. Basically, it is the result of negligence. Either inexperience. But there is a kind of error that we call intentional. It's kind of a signature. "The stupid sin of the devil is vanity". What a joy to be a devil if you are unable to talk about it?
A killer normally commits twenty errors, but is only aware of a third of them. Most of them are due to carelessness or inexperience. But there's one type of error that we can consider deliberate. It's a sort of signature. "The devil's dumbest sin is vanity". But where's the fun in being the devil if you can't let anyone else know?