How does it fit into the film as a whole?

This film is about a "demon barber", so in the opening scene, it only make sense to include blood dripping off barbers chair as of course, Sweeney Todd killed men in revenge of one man, years ago, taking him away from his wife and baby. But in the whole movie, Todd has his eye on only one person to kill. That person is the Judge. The one who has Todds' 16 year old daughter trapped inside a room day in day out. That's where Mr's Lovett comes in. One who's business isn't going so well as meat is expensive their days. Mr Todd and Mrs Lovett work together to keep their businesses up and running by Mr Todd killing his clients by slitting their throats with knives which were used as razors, then Mrs Lovett makes the dead men into pies and sells then locally. The whole movie is an evil, creepy, and deranged story. Therefore the opening scene sums up the whole storyline in a very short space of time.