One of the greatest things about humans--perhaps one of the only things that sets apart from other animals--is that we take abstract ideas very seriously. It's also one of the worst things.
Something like tribal warfare is known in chimpanzees, and there was a recent discovery of the remains of a possible tribal war between non-agrarian humans 10,000 years ago. So we're not unique on that score: murder likely ran in the blood of our last common ancestor.
But at some point one of our ancestors believed that it was necessary to kill another human--not because of a threat to self or family, or for control of a desirable resource, but because of heterodox belief about a thing that did not exist in the real world. Probably, sometime around then, some human reckoned that it was worthwhile to risk its life for an abstraction. Before this time, these thoughts had never ever occurred. There was a first time.
Possibly, about the same time as those humans were killing and being killed, some residents of Anatolia were carving and erecting monumental stones at what is now Gobekli Tepe--making a considerable sacrifice of time, energy, and wealth, to an abstraction long forgotten. That's the only material evidence we have of this development, and it is highly enigmatic.
We'll never know the first belief that inspired murder, the first abstraction that instilled a willingness to die. We can't ever know how many millions of humans have lost life to ideas, how much suffering stems from notions without substance--nation, sect, dogma, politics, racial purity--since then. It's been about 10,000 years since this idea appeared. Sometimes I let myself hope that we will grow out of it; that at some point, maybe less than 10,000 years from now, the last murder will be committed for an idea, the last sacrifice made for a belief. It will, in all likelihood, be well after I die.