Rockefeller forced his workers to work long hours and with low pay was bad because it tired them out and they wouldn't be able to see there family and spend time with them, also if they were not making enough money they would have to send mom to work then the kids if they were not making enough money.
Also Rockefeller is a robber baron because he discouraged union activity this is bad because he wanted to pay his workers what he felt like and how much time and effort they put into the job.. His employees pointed out that he could have paid his workers a fair wage and settle for being a half-billionaire. This shows how much power and money he wanted for himself