this is silly. antitrust came from the Standard Oil days. the name itself is in reference to Standard's novel approach of using a trust as the root of its organizational hierarchy. notoriously, Standard Oil dominated the market by vertically integrating for efficiency, and then telling any competitor in the geographical area they were about to expand into "either merge with us on really shitty terms, or we'll sell at-cost until we kill your business (because at-cost for us is in-the-red for you)."

Standard Oil would be labeled "pro consumer" by today's crowd: cheap prices, and highly standardized products that you could rely on wherever you travel (like McDonald's or Starbucks is today). the notion that antitrust can be boiled down to "consumer harm" is new. the history isn't vague, and thereby neither is the "theory". the legislation, i'm not sure about, but since it's rooted in those two things and vagaries are easily cleared up by consulting the former.