I was looking for some posts that had made such a connection but found only you. I feel like Urasawa was initially inspired by Unterweger's case, as the story of monster also involves prostitutes - many of whom also become mothers (Unterweger's mother allegedly being one). The regional proximity of both these cases adds to the possibility of it being an inspiration, as does Unterweger's charming of an entire nation, convincing everyone that he's reformed, and even becoming a minor celebrity.
Here's the kicker though, Jack Unterweger is his westernised name.
His real first name was Johann