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 wanted to fix it this way:
Quang wants a degree in business administration,
because he fantasizes about yelling at future subordinates who are
too afraid of losing their jobs to defend themselves.
When you have two main clauses joined with just a comma, you can fix the problem by subordinating one of them. Because is a subordinate conjunction that will do this job.
When because comes after a main clause, however, you do not need a comma with it. An important rule to remember is that 99.99% of the time, there is no comma before because.
You might want to consult the rules for fixing comma splices and fused sentences.