Huey Long was seen as a kind of fascist menace that could have taken over the country. He was to the left of the New Deal, and many saw him as a kind of radical. He was also seen as someone who could begin mob rule. Since the US didn't have a real fascist organizations or leaders, Huey Long was the closest thing to it - a charismatic man who could start a mass radical movement.
If Long was merely a local Louisiana politician, he'd only be seen as just another corrupt Louisiana politician. But Long was also a national figure with a national following, and that scared people from other states where Louisiana style politics and corruption was not the norm. The other states had their own corrupt politicians, but it was different in degree and kind to Louisiana. If you from California or Kansas or Massachusetts, the idea of someone like Huey Long gaining control of the Presidency scared the hell out of you.
I don't know why people think it is absurd that bad people can also do good things. That Long was a populist who achieved some social improvements does not mean he was not a threat to the normal workings of a democracy like the rule of law. A lot of Europeans refuse to classify Hezbollah as a terrorist organization simply because it runs some charities. Hitler and the Nazis actively promoted a lot of "progressive" policies and ran charities like Winter Help and invested a lot in good things. Doesn't make them less evil. Hugo Chavez was able to temporarily improve certain social conditions in Venezuela en route of building his dictatorship and subsequently destroying the country.