Dear Children: Based on the graveyard conversation in chapter 1, The Crooked Saint seems to be a locally infamous serial killer. There are few details given about him, but it is implied that the bodies of his victims were heavily mutilated.
The Dragon Doctors: The Doctors confronted a magical serial killer in their "Messenger of Death" chapter. The murderer killed several shamans using summoned spirits of his own; finding out what spirits were used to determine the character profile of the murderer was the main focus of the plot.
Achewood: Nice Pete, who combines type 1 (visionary) and type 4 (power/control) with a thoroughly creepy dead-eyed stare.
Jared features four serial killer characters. Mary and Shia are more Headonistic, while Lilac seems to like the power. Jared is more of an antisocial killer.
The Greatest Gift What Happens In Hayagas presents a chilling version in Venus (or rather Jupiter, who took her twin sister's name). She spends the story arc trying to personally murder Inidouh's entire family, starting with Hazel. At first we think she's doing this because Inidouh rejected her, but then we find out that her 'when I don't get what I want, ponies suffer' comment was far worse than it seemed. The investigation of her mansion after she's killed reveals that she had a list of ponies, all of whom that had been checked off were dead or missing. What makes her so chilling is the fact she outright says killing Hazel (actually a decoy) wasn't as satisfying as she thought it would be, all the while keeping a completely serene look on her face.
Dongsoo Seon from Bastard!! has been serially murdering women for years, going so far as to use his son Jin to help him do it, initially as bait and later, when he's older, as an unwilling accomplice.
Everyday Heroes: In Jane's origin story, it's revealed that Jane's boss, Wrecking Paul, was a female-targeting killer, who used their heists as cover for murdering female guards and heroes. And, as Jane tragically found out, his own henchwoman, in a pinch.
Clockwork: Arcadia has been plagued with at least one, perhaps several in recent years. Their aristocrats have been ruthlessly targeted, and they can't seem to catch the killer(s).
In the Mystery Babylon prequel comic Kick Girl, Kill Boy is wanted as a serial killer known to the press as the Stalker of Babylon. He is a supernatural killer of the hedonistic / antisocial type.
Several people have called the mercenaries serial killers over the years; the mercenaries usually just ignore them and keep doing their jobs. Occasionally they leave behind the corpses of the murderers that they just saved everyone from.
Tagon: We could just leave that here as a reminder of how to put power behind the voice of the people.
On a (slightly) more serious note, Tailor, a robot given to Captain Tagon by his father, is worried that Tagon is going to destroy him, and brings up his concerns with the doctor.
Tailor: His eyes promise only death, and his words are worse! Is he some kind of serial killer? Bunny: A serial killer? No, if anything, he's massively parallel. Tailor:[deadpan] Oh. Okay.
When Para's freed robot LOTA comes into possession of an incredibly powerful, unblockable superweapon, Petey warns LOTA that he is building his own weapon in case LOTA becomes a massively parallel serial killer.
Notably book 11, the book where LOTA gained control of said superweapon, is titled "Massively Parallel."