I think they're saying it was a single event because (so they also say) all 37 people were all killed in the same way: close-range, with a blunt instrument, and at what they assume—from the carbon dating and location of the bones—was about the same time. They do jump to the conclusion that it was a massacre pretty quickly, but it does make enough sense, I guess.
They speculate that cannibalism could have been involved, but don't talk about a serial killer. It seems a stretch for someone in a Bronze Age society to secretly commit 37 murders, followed by systematic butchery of the bodies, and then burial in a common grave. 37 people may have been the majority of people in your tribal unit, so it would be kind of hard to kill them all without someone noticing!
Why would it be a stretch? Again, you’re making a lot of assumptions. We don’t know a lot of how people lived then. Serial killers in modern society have committed brazen crimes in small communities and got away with it for long periods of time. It seems not very rigorous to discard alternative explanations. I’m not saying it’s likely or the probable thing, I just am not really convinced they were very rigorous about any other possibility other than the conclusion they set out to believe in the beginning.
I agree they didn't give equal time to all possible explanations, but given that they can only speculate, I think they probably chose the most realistic explanation from among the likely possibilities, and they gave it enough throat-clearing up front that I wasn't confused about whether they were saying they'd proven it or not. I'm not saying they couldn't have done a better job of saying "hey, we're just guessing here, for funsies", but it didn't bother me.
They speculate that cannibalism could have been involved, but don't talk about a serial killer. It seems a stretch for someone in a Bronze Age society to secretly commit 37 murders, followed by systematic butchery of the bodies, and then burial in a common grave. 37 people may have been the majority of people in your tribal unit, so it would be kind of hard to kill them all without someone noticing!