A funny thing is, Randall IS twice the scarer Sullivan is.
Ok, some may say “Randall is Nexas’s fave”…but hear it out. The first time Randall tried to show his stuff as a Scarer, Sullivan ruined (accidentally) it for him. In MI, Randall’s tired and fatigued from the events with the scream extractor and an elevated stress level due to Waternoose and Fungus. Yet he STILL manages to match up to Sullivan…and he’s NOT at his best to do it. (Sullivan is a fit individual, as he frequents the gym and has a trainer (being Wazowski) to keep him up. Randall doesn’t really have that with Fungus.)
Remember that line. “banished my TOP SCARER”. Not that “I just had to banish James” or even “banish Sullivan”. Waternoose shows he has his pawns and his prize winners. Sullivan seemed to actually be more of a prized show trophy, which goes for MI’s image (commercials, products, reputation increase). Sullivan was an ideal front-man for the company’s workforce. Waternoose may have seen Sullivan more as a respected pawn that he’d rather not throw away if he could help it. There could have been other ways to handle the situation, but Waternoose chose the quickest and easiest, getting rid of the two.
Some, like PBL, would refer to a certain psychological condition of Randall pandering to Waternoose as a form of a father figure, but let me just go simple…
Waternoose is a manipulator and knows how to handle a person. He does it positively with Sullivan (with several grandfatherly gestures) and negatively with Randall (barking orders at him and insulting his progress). He KNOWS them. He knows Sullivan is an everyman and respects Waternoose, allowing Waternoose to count on Sullivan for things (such as demonstrations) and some confiding (the shareholders), and can exploit it. Same for Randall. He knows Randall’s a hard worker, that he aims to be respected and make something of himself. Randall wasn’t a “bad guy”, as some would put it, from the point of his unfortunate college year to MI. He became a scarer, a great scarer, a top scarer…he’d gotten a taste of the stuff he felt he needed. So when he lost it (theories suggest unfairly, again), he became the perfect pawn for Waternoose to manipulate.
That line was about driving in how Waternoose really felt. Randall was only good to him, at this point, as a pawn to revolutionize the scream industry, not as a scarer. He KNEW that Randall and Sullivan had a past (far more than seen in film), that Randall believed Sullivan had cheated himself to the top. It was the same as saying “that CHEATER is better than YOU and you will NEVER be better than him”. That REALLY irked Randall to the point of displaying his distaste for Waternoose.
“Sullivan got what he deserved” - Randall’s line the moment before, rather solemnly rather than a gloat or egotistical response. He believed that Sullivan deserved what he got, but Waternoose, the one who DID the deed himself, brandishes otherwise. Randall was doing so much (risking personal health, keeping things quiet, working on the machine, keeping up his own scare total, etc.) that it really ticked him off that the person he was doing all this stuff under just says he’s below a cheater, that somebody that’s WORSE is actually BETTER than him.
It’s a real ninja-move to things and may have helped spark Randall’s descent into that temporary mental break we see shortly after. A running drive of “he’s not better than me, people like him shouldn’t get the respect and affection of people”.
I could probably word the theory better at some time, but it’s been a long day.