The Magnus Archives: the episode "A Father's Love" is narrated by a serial killer's daughter, who describes spending her childhood under his care as, unbeknownst to her, he commits his murders.
Terrence of KateModern is revealed to be a serial killer in "Precious Blood".
Unexpectedly used in Survival of the Fittest. Johnny "Mordread" Lamika of version one and Walter Smith of version two were both serial killers despite being high-school age teenagers. Johnny was explained as having initially never left enough evidence for the deaths to be considered anything but an accident, but was eventually caught and sent to a mental institution after a successful insanity plea on the part of his defense. Walter got away with his actions because his father was a Senator, and used his influence to cover up Walter's actions and get him out of trouble on the few occasions he was suspected. In return, his father would sometimes bring him political enemies to torture and/or dispose of.
Cause of Death is all about this. The very first episode shows the nameless killer bust into the victim's house and kill him... with a granola bar. There seems to be some mystery behind who the woman is in the picture, but the story is ongoing.
The Heaven Cycle has Ashton Sharpe, who had a habit of purchasing victims from a human trafficking ring so he could rape them, and then torture them to death.
Worm features the Slaughterhouse Nine, a gang of thrill killers with high-end superpowers who rampage across North America attacking cities and subjecting random people to hideous torture and carnage. They likely have a higher body count than the rest of this page combined; their visit to Brockton Bay kills hundreds inside of a week, and they've been active for more than ten years.
Jeff the Killer. He was driven insane and killed his family before going on a killing spree. Arguably the most well known creepypasta example of this trope.
RedLetterMedia's Mr. Plinkett is a 119-year old serial killer who murders women in his creepy basement. He's also a very opinionated movie critic.
The "Heavy Hitter" episodes of The Last Podcast on the Left are spotlights on serial killers throughout history. Typically running three episodes long on average, every profile follows a formula of describing the central figure's childhood, with the first episode typically concluding at their first murder, the middle episode(s) going into their careers and how they avoided being caught, and the final episode describing their ends. They do all this while treating the horrible people involved with all the respect and gravitas they deserve: Absolutely none.
Migraine reveals that Ken Muntz is one, having already killed at least seven women using the same M.O. with each victim.
My Dad's Tapes: The father, donotcontinue, and Uncle Don each have a massive body count, mostly made up of young women. The Zodiac Killer is also mentioned, having been the father's mentor.
The Human Pet: Sam Deercot had been kidnapping, torturing, and killing people since at least the 70s.