They were willing to do everything up to and including murder. For Rockefeller you can simply look at the Ludlow Massacre. It occurred at a coal mine managed by Colorado Fuel and Iron company, which was owned by the Rockefeller family (though the actual operation was run by John D. Rockefeller's son). The cola miners attempted to unionize and threatened to go on strike.
Their demands (Copied from wikipedia because I'm lazy and don't want to type them out)
Recognition of the union as bargaining agent
Compensation for digging coal at a ton-rate based on 2,000 pounds [20] (Previous ton-rates were 2,200 pounds)
Enforcement of the eight-hour work day law
Payment for "dead work" (laying track, timbering, handling impurities, etc.)
Weight-checkmen elected by the workers (to keep company weightmen honest)
The right to use any store, and choose their boarding houses and doctors
Strict enforcement of Colorado's laws (such as mine safety rules, abolition of scrip), and an end to the company guard system
Hardly unreasonable wouldn't you say? And I think speaks to the other unscrupulous business practices utilized by men like Rockefeller you asked about.
In response Colorado F&I evicted the miners and their families from company owned housing. The company then brought in non-union workers as well as the Baldwin-Felts detective agency to protect them and to harass the strikers. This harassment involved the reckless firing of a machine gun mounted on a car called the Death Special into the camp as it patrolled the camps perimeter. This could result in injuries or deaths. Later the body of a non-union worker was found. The company and a sympathetic National guard commander used this as an excuse to attack the camp. They set up a machine gun on a ridge overlooking the settlement and a gunfight broke out. Both sides have claimed the other started it. The National Guard then set fire to the camp (EDIT: /u/Bodark43 pointed out there is disagreement about how the fire started and it may have been an accident.) In the aftermath death tolls varied but included multiple miners shot to death as well as two woman and eleven children who burned to death/asphyxiated in the fire.
Source Not a historian but this is the book I read
Rockefeller and Standard Oil demanded rebates for shipping oil on railroads. They also demanded rebates from railroads on oil shipped to smaller, competing oil refiners. Rebates were outlawed by the Elkins Act of 1907, that placed strict regulations on the freight rates the ICC allowed railroads to charge most industries.
