O'Doherty-Dangerously Delicious Meat Pies

Although Sweeney Todd and Mrs. Lovett are dark, mysterious, creepy murders, viewers tend to sympathize with them instead of their victims. It may sound scary, but viewers actually relate more to Sweeney and Mrs. Lovett than they do with the rest of the characters. Because even though they kill people, they see the world for what it truly is...The story takes place in London during the Victorian Ages. The story takes place in a high urbanized area-- complete with violence, crime, corrupt leaders, and inhumane working conditions.  
With that said, it is easy for Sweeney to overcome moral revulsion about murder and cannibalism because he sees "man devouring man" on the daily. There is always chaos and crime in the streets, the government officials and people in other high level positions who are supposed to serve the people are actually thieves who use their power solely to benefit themselves, and even the people who are of middle and lower class tend to live by the motto "every man for himself." Everyone takes what they can and they give nothing back. Nobody really gives a rat's behind about one another. 
Another factor that allows Sweeney (and Tim Burton) to justify the murder of relatively innocent people is the fact that London is a highly industrialized city and all the people who live in it are a product of industrialization. Thus everyone is the same...so it does not matter if a couple people are killed here and there. The people Sweeney killed did not have any unique gifts or significant qualities, talents, ideas, etc. to offer to the world. They lacked passion, creativity, and imagination. And everyone who did have any of those things they condemned. Thus the world had not lose anything when they died. In fact one may argue that the world actually benefitted from their deaths. One obvious reason people benefitted was because the bodies supplied people with a good source of protein (meat was very expensive in those days after all). But another reason the world may have benefitted was because many of the people that were killed were powerful, corrupt, selfish, and just plain evil. In fact, Sweeney believed that they were getting exactly what they deserved.
Although cannibalism does not happen often in real life, cannibalism is an important element in this film. In real life, people harm and figuratively "devour" fellow human beings, but we usually do not think of it that way. Perhaps this is why Sweeney's story is so disturbing-- it present to us, what we do unconsciously every day in a literal manner.