From my perspective I wouldn't consider Dennis to be a bad manager, he is someone who I can tolerate working for, although I didn't enjoy seeing Dennis terminating Brad I can see his reasons why he would terminate him.
Dennis fired Brad because Dennis was afraid of the asshole customer making a stink to corporate. What he should have done was told the asshole to blow and never come back. There were no security cameras back then - Dennis could have told corporate that the guy was constantly abusive to his counter help and always demanding refunds.
Dennis was a dick of a boss, to humiliate a good employee for the benefit of an asshole customer instead of standing up for him. Unfortunately, bosses like that are not rare, especially in minimum-wage jobs.