For 99.9% of human history we lived as hunter-gatherers, in small family groups of 5-20 individuals that came together a couple of times a year into groups of a few hundred to perhaps a couple of thousand. You were related to and personally knew everybody in your clan group. Anybody outside of your clan group and territory was your sworn enemy and would kill you on sight and vice versa. At this stage serial killer was a meaningless term. Anybody who wanted to kill a stranger simply went on a raid into enemy territory and killed someone. It wasn’t an action that was questioned, though the chances of getting away with it multiple times was probably not high.
Following the invention of agriculture people started living in towns and cities. Even at this stage, the scope for serial killing was limited. There weren’t that many cities, and most were still only ten thousand people or so people. In a city of that size the chances of a serial killer being able to operate unnoticed even today is remote. Even in the larger cities, the nature of the society made serial killing difficult to sustain. Everybody but the ultra-rich slept 5 or more people to a room. Very little happened after dark due to the expenses of lighting, so any noises at night drew a rapid response. Unlike today, group infrastructure was paramount; everybody was expected to know all the goings on of their entire group. So the scope for a serial killer was limited. You couldn’t gab people off the street, you couldn’t readily find people to invite home to kill as Dahmer did, and even if you could, your neighbours would know all about it.
There may be a reason why serial killers became more prevalent following the industrial revolution. As people began moving to cities looking for work the social group began to break down. Secrecy became easier and the increase in nighttime activity made targets easier to find.
I don’t doubt that there were people who were psychologically serial killers in ancient times, but it would have been much more difficult to get away with multiple murders because of the way the world was. People disappearing were more likely to be noticed, and any unusual activities were also more likely to be noticed by the neighbours. None of this means that they didn’t exist, just that they were unlikely to run up a score of dozens of victims before being caught.