Oh it was before Grindelwald went on his reign of terror, mind you. They were childhood friends. Grindelwald had family near where we lived and visited for the summers. They were both skilled wizards and pretentious pricks. Thought they could fix the wizarding world and make it better in the way they thought it needed to be done because they were bloody full of themselves. Didn't matter that they were a couple of boneheaded teenagers. They thought they knew best, they'd always thought that.
They split before Grindelwald really went dark, but my point is some of these folk probably align cause they think its right for the world, or it helps their family standing. Course you get a couple of them, like the Lestranges, that really buy in and are zealots. But people have been doing shitty things, even the ones that people hold on a pedestal, because they think its right for a long time.