Take a look at the original sentence again:
Quang wants a degree in business administration,
he fantasizes about yelling at future employees who are too afraid
of losing their jobs to talk back.
You chose to fix it the correct way:
Quang wants a degree in business administration
since he fantasizes about yelling at future employees who are too
afraid of losing their jobs to talk back.
Quang wants a degree in business administration is a main clause. He fantasizes about yelling at future employees is the second main clause. One good way to fix two main clauses that are incorrectly joined is to subordinate one of them with a subordinate conjunction like since.