Randall has always been a manipulative, spoiled asshole. Rebecca and Jack always put his needs ahead of their other children. When Randall began having OCD and/or anxiety issues they bent over backwards to placate him. He got so much more attention than his siblings. Jack did the "daddy's little girl" thing with Kate, so she was got special attention. However, Kevin was really ignored by his parents because he rarely demanded their attention. Randall steamrolls over his parents, siblings, his wife and kids. He wants what he wants and manipulates everyone around him until he gets it. Look at what he has put Beth and his girls through.
Bringing William ( his bio dad but a complete stranger) home without discussing with Beth first.
After William dies, he buys his dead bio dad's apartment building.
He impulsively quits his high paying job.
He decides that he and Beth need to be foster parents.
He decides to run for public office two hours away from his home. This office pays much less than he would be making if he had not quit his job.
Beth loses her job and decides in a midlife crisis career change.
The family is overwhelmed by Randall's commuting 3-4 hours a day. Beth does not demand that he resign his office, she decides to uproot her family's life by moving to Philadelphia.
Beth did not really fight against Randall's self-indulgent whims because she knew he would never back down because he always gets his own way.
Now Randall is emotionally blackmailing his mother into doing a drug trial halfway across the country, taking precious time away from her family. It does not matter to him what Kate, Kevin or her husband feel about this, it is all about him. Randall Pearson is a miserable excuse of a son.