Review the original item:
Quang wants a degree in business administration,
he fantasizes about yelling at future subordinates who are too afraid
of losing their jobs to defend themselves.
You chose the correct way to fix the error:
Quang wants a degree in business administration since
he fantasizes about yelling at future subordinates who are too afraid
of losing their jobs to defend themselves.
Quang wants a degree in business administration is a main clause. He fantasizes about yelling at future subordinates 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.