As serial killers go, the mountain man John Jeremiah Johnston may hold the record. He is said to have murdered around 300 Crow Indians in the 1840’s and ‘50’s. He blamed the Crow for murdering his wife, a Blackfoot Indian, and swore that he would take revenge on the whole tribe. Later, he joined the Union Army during the Civil War and wound up in Santa Monica, Calif. where he died in 1900.
I don’t know if that qualifies under the modern definition of ‘serial killer’..................
The movie Jeremiah Johnson is “family entertainment” version of the story. He was referred to as “Liver Eating Johnson”, because that what he did to his Blackfeet victims.
https://en.wikipedia.org/wiki/Liver-Eating_Johnson