The word is built from the numeral hundred and the word for an ox, and it was already loose in its earliest use. In Homer a hecatomb may be of bulls, of sheep or of goats, may number twelve or fifty or a hundred, and the phrase functions much as a great sacrifice would in English. What it always means is scale: the largest offering the occasion can bear, brought by a community or by a king on a community's behalf rather than by a household.
In the historical cities the scale is real and measurable. Great festivals killed animals in numbers that required organisation of a kind that has nothing sacred about it: contracts for the supply of beasts, officials to inspect them, space and cauldrons to cook them, and a scheme for dividing the cooked meat among a citizen body of thousands. At Athens the meat of the largest festival sacrifice was distributed through the demes, so that a citizen received his portion as a member of a named community rather than as an individual in a crowd, and non-citizens received either a lesser share or none.
That is the aspect of the hecatomb worth holding onto. A Greek city had no other occasion on which it fed its whole population at once, and meat was otherwise rare on most tables. The offering to the god and the distribution to the people were the same act, and neither can be described as the pretext for the other without distorting what the sources show.