Why is Bigtime so abusive towards his brothers? Simple, the only way he knows how to treat people is from how Ma Beagle treats people... which is to say horribly.
"Junkyard Stew" seemed to be a setup for bad cooking from the Beagle Boys, only for Huey to find it pretty good. How can this be when the Beagles clearly were raised in an actual junkyard? Because they're better off than they seem. "McMystery at McDuck McManor" seems to indicate that Ma Beagle has a shadow business that hides their thieving ways as a "legit business woman". So despite needing to provide for her large family, she did know how to use the finer ingredients like cumin to make it better.
Each of the brothers needed exactly one of their other brothers' personality trait to solve their own problem: Dewey needed Huey's intelligence to fix the robot, Louie needed Dewey's courage to stand up against Doofus, and Huey needed Louie's charisma to convince the Beagle Boys to stop following him.
This isn't the first time that Doofus gets corrupted when gaining a measure of power. In the original series' Superdoo episode, Doofus gained Powers for a Day, and became a Smug Super, only getting back to his old self when he has a Jerkass Realization. Here, instead of physical power, Doofus gained an impressive monetary one, but, unlike the 80s Doofus (who was friends with the triplets, and had good role models in Scrooge and Launchpad), he was a lonely child who didn't have friends or good parental role models (they obviously didn't know how to educate and control him before it was too late), so he became completely corrupted by his power.
Huey just taught the brainless Beagle Boys some practical scout skills. Who's to say that they won't use those against him and his family in future episodes? They already used some of them against Bigtime.
