Domin is the boss. That's his thing. His name even has the same root as "dominant." He runs the R.U.R. factory, sending important messages this way and that way, ramping up production, and dictating to his secretary. He's got the power of life and death—when Helena accuses him of lying about Sulla being a robot, he just sends her off to be dissected. He makes Helena marry him, even though she never consents. "You are—you are—oh God!" she says in exasperation, and he replies smugly "There you see?" (prologue 391-392). He is a God; he makes life like God and transforms the world as he sees fit.
Domin says quite clearly that his goal is mastership, and divine power—for himself, perhaps, but also for mankind in general:
To hell with the dividends! Do you think I'd have worked even on hour for them? [He bangs on the table.] I did this for myself, do you hear? I wanted man to become a master! So he wouldn't have to live from hand to mouth! I didn't want to see another soul to grow numb slaving over someone else's machines! (2.60)
Mastery and power for Domin, then, means not being a worker. It means having control over the machines, rather than being controlled by them. Domin wants power so that people can be human and free. You must dominate to be the master; otherwise you are the slave.
So, you could argue that Domin is trying to do good, even if he comes across as a jerk. But, alas, he doesn't just come across as a jerk—he comes across as a mass murderer. When Helena points out that robots have been used in war to kill huge numbers of people, Domin says that it wasn't a mistake at all.
"We predicted that, Helena. You see, this is a transition to a new system." (1.115)
Domin figured there'd be a massive world war, and he was cool with that because he figured you have to break some eggs to be the master.
And what does breaking all those eggs get him? Mastery? No, not really—instead, it results in creation of all those robots, who have their own ideas about who should rule. "I do not want a master… I want to be the master of others… I want to be the master of people" the robot Radius declares (1.245-249).
Where did he learn that? Where else but from his creator, dad, and master, Domin? Radius is really just Domin all over again. And when Radius takes over, shouting "There are no people. Robots to work! March!" (2.394), he might as well be Domin, who never treated anyone like human beings either. Domin imagined a world in which he could treat everyone as things—and inevitably, he gets treated as a thing himself. If you insist on seeing the world in terms of masters and slaves, the play says, eventually, sometime, somewhere, someone will master you.
This is maybe overly optimistic; outside of fiction, jerks like Domin treat people like robots and die happily in their beds. But it's nice to imagine a robot apocalypse administering justice, even if you don't always get the satisfaction in real life.