Pedro Rodrigues Filho
I’m kinda iffy on this guy. On one hand, yes, he was a serial killer. On the other, he was a true-to-life Dexter Morgan. Almost all of his victims were from the criminal element, or had done something to hurt someone.
During his career he murdered a corrupt politician, his own father for abusing and killing his mom, and several drug dealers, all before the age of 18. By the time he was convicted and arrested in 1973, he had 71 deaths under his name, and then went on to add another 41 in prison.
He was released in 2007, and no one’s heard from him since.
You forgot Harold Shipman, probably the world's most prolific serial killer and a doctor.