The book I'm currently reading, Popular Crime
by Bill James, points to a couple incidents in the mid-60s - the Texas clocktower incident was one of them - that kicked off the whole mass murder thing. Before then, yeah, there really weren't a lot of them. That's a little problematic though because
a. A big part of that was that before the 1920s most people didn't live so close together, and
b. A *lot* of crime, even homicides, especially homicides, went unreported before the Second World War.
I think I have a pretty good handle on what life was like in the USA 100 years ago and although I can't point to any stats, I am almost positive that not only were crime rates higher (homicide rates too), but they were a *lot* higher, exponentially higher than today. If someone during that time started killing prostitutes or hoboes it was virtually impossible to determine whodunit, let alone convict that person, and so police generally didn't even bother to investigate those cases. Likewise, cases involving gangs, and the big cities had gangs that dwarf the ones we have today. This is also an environment when a woman who carried typhoid fever could infect a dozen people, get found out and quarantined and then go right back out and find several other jobs as a cook, infecting dozens more and killing Lord knows how many of them in the process. At around this time, someone walked into a bakery, poisoned a bunch of pies, killing at least one woman, and was never caught. Really, I think that the old policeman's saw that the vast majority of homicides are committed by people you know came into being because of this - at the time, the only time they could ever actually catch anybody is if they were somehow related to the victim.
It's often said that scandals respresent an increase in morality rather than a decrease in it and I think this is no exception. 100 years ago if a white Hispanic had shot and killed a black kid for carrying a packet of Skittles and an ice tea, nobody would have thought anything about it, and if you pointed out that the local police were dragging their feet in prosecuting the case, you'd probably be laughed at. Scott Peterson probably would have gotten away scot-free without a body and few outside of Laci Peterson's family would have batted an eye at this. Cases like these make the papers nowadays because of their extreme unacceptability.
That being said, there were some high-profile mass murders before the 60s...http://en.wikipedia.org/wiki/List_of_rampage_killers
One that's not listed for some reason was a case in I think the 1920s where a school official set off a bomb in the school he worked at, causing it to collapse and killing many of the children inside. When the authorities went to his house to apprehend him, they found him sitting in his car, which was rigged to explode as well.