"Just to clear this up, Sweeney Todd was a real person. This character is just loosely based on the real person. The real Sweeney was simply noted as "a cold blooded murderer with barely any motivation for his wrongdoings."
He then appeared in Penny Dreadful stories, and was only altered a little bit. The musical changed a lot, giving him an entire motivational back story.
However, he was a real person, and so was Mrs. Lovett.
Don't believe me? Well, look up the silent video clips from 1820's London. A couple of them actually show Sweeney's Barber Shop and Mrs. Lovett's Pie Shop. However, they were not even next to each other. They were simply both on Fleet Street & it is said they may have had some sort of affair.
My point: Sweeney Todd the malicious murderer was real.... Sweeney Todd the demon barber of Fleet Street was based off of the malicious barber, but not fully the same as the real man. :)
Get over it, people. He was real."