tl;dr - religious activity matches the emotional needs of people based on their dominant economic activities. Human sacrifice most commonly appeases the spirit of the crops killed and is associated with early agriculture.
Human sacrifice is associated with societies that have recently adopted agriculture. ( I think Mr. Henrique & I can agree that Rome's adoption of a agriculture was not recent; Rome was formed from societies that were already established as agricultural. )
Couple of notes based on comments
"post agricultural" does not mean that people stop growing food. I'm very hesitant to state the following because I'm summarizing a complex topic, and the words aren't used in a colloquial sense - I don't want to offend.
Forage societies are organized in very small bands - generally < 25 people. The social organization tends to be participatory democracy or "big-man". They are almost never literate. (AKA Hunter-Gatherer) Religious practice is appropriate to the economic activity (hunting & gathering)
At some point many cultures develop horticulture or agriculture - social organization tends to shift to a Priest-King dynamic, and social unit size expands into the hundreds. Religious activity changes to an organized priesthood (necessary to organize the activity of 10-100x more people operating in a social unit). This is where human sacrifice is most common - men must die to appease the fertility gods. I'm vastly oversimplifying; the material is very easy and very available, but you're studying anthropology, not history). Literacy appears at this stage.
At some point commercial and industrial activity emerges; there is another shift in social organization, in religion and in size of political units. Rome is in this stage; their religious activities are appropriate for their economic activities. (and although the majority of inhabitants in the Roman Empire may have been engaged in politics, those individuals had almost no civil rights - they were slaves. They had religious practice - it is a different question for a different forum if that practice was "pagan". The Romans who were involved in social organization and power the religious and political leaders had nothing to do with agriculture - they owned land and slaves, and may have grown cabbages as a hobby, but their religious questions had nothing to do with the guilt over killing corn. The rules around the Roman Pontifex had nothing to do with harvest - they had to do with preventing him from touching weapons of war and scheduling festivals for the populace.
This pattern isn't about east-west or north south or climate - it is about the emotional/spiritual needs of the people engaged in economic activity. I'm doing a horrible injustice by summarizing huge bodies of theory in a couple of paragraphs (and I haven't researched this area in 20 years, so there is doubtless new theory available).
Most important takeaway - ignore everything else I've said and just go read some of the material cited below. It is accessible, fascinating, eye opening, clearer than my summary and you'll be glad that you did.
I can't provide a pithy quote, but you may find it useful to consult
- Joseph Campbell on myth
- Bill Moyers on Joseph Campbell on myth
- Fukuyama History of Political Order (not obvious, but it is there).