Republicans equate being pro-market with being pro-big-business-agenda. The assumption is that anything that is good for big business is good for the market and therefore good for consumers.
So in the Republican framing, anti-trust, since is interferes with what big business wants to do, is *necessarily* anti-market and bad for consumers, which if you accept their axioms would have to be true, even though what big business wants to do is use its economic scale and political clout to consolidate, evade competition, and lock in consumers.
That isn't economics. It's religion. And when religious dogmas are challenge, you call the people challenging them the devil -- or in current political lingo, "terrorists". A "terrorist" in that sense doesn't have to commit any actual act of terrorism. He just has to be a heathen.