Just lost a run with a rouge setup on the classic map (the Pokémon lookin one), I had 8 kills and serial killer never triggered causing a loss. I believe the fire damage can interfere with the serial killer boon so not sure if that’s considered a bug or just an annoyance.
#Flamewalker interferes with serial killer
strange geyser
hushed perch
I wouldn't be surprised if a kill from flamewalker does not count towards a Serial Killer kill. I did propose an alternate wording to SK's description.