He's a really interesting guy, but wasn't our first serial killer. We've had so many crazy killers. The Glanton Gang murdered the hell out of Indians and Mexicans, for instance. John Wesley Hardin killed people of all persuasions. Any number of bushwhackers during the Civil War probably outpaced Holmes. That mountain man whose name I can't think of that killed and ate Crow Indians.
Delphine LaLaurie in particular is a nasty piece of work, torturing and murdering enslaved people in New Orleans.
An unsurprising aspect of American serial killers in the 18th and 19th centuries is that they only gained notoriety for killing white people, like Holmes did. King Fisher is a self-reporting example of this phenomenon, claiming to have killed X number of men, "not including Mexicans."