I remember our biology prof showing us a chart relating relative penis size in apes, to, I think it was monogamy (or maybe peacefulness) ? There was an inverse correlation. Humans have relatively big dicks compared to other apes, which would seem to indicate humans aren't a traditionally monogamous species.
Take this with a major grain of salt, that class was 20+ years ago.
"But in that kind of world, this type of murderer wouldn't exist in the first place."
Yes, thank you for saying this.