Was hatred of Standard Oil irrational?
Actually, it was somewhat.
In 1865, the price of kerosene was 58 cents/gallon and Standard oil had almost no share of the market. By 1870, Standard Oil had a 4% share of the market and kerosene prices were at 26cents/gallon. In 1880, Standard had a 90% share of the market. Kerosene prices were now at 9cents a gallon. After a decade of 90% market share, kerosene prices were down to 7cents/gallon.
Rockefeller did such things as purchasing entire forests so he could make his own barrels. The result is a barrel price drop from $3 to $1. Rockefeller also offered guaranteed daily traffic to the railroads using Standard-owned cars, loaded and unloaded in Standard owned facilities. The result was a lowering of transport costs from $900k per trip to $300k per trip.
When it came to take-overs of competitors, Rockefeller opened the books and made a reasonable offer as he wanted talent and assets. If they refused, then he would start undercutting on price (while still turning a profit).
Now Standard Oil wasn't broken up until 1911, but due to competitors copying Rockefellers methods, its market share was at 65% and falling. Standard Oil didn't stop competition, it only forced them to become better.