He was the Voldemort before Voldemort went all nasty. Grindewald went evil, and the wizarding world asked Dumbledore to step in and take him down. Dumbledore is famous for this battle. Also, it was implied in later books that Dumbledore and Grindewald had a relationship. Of the sexual kind. J.K. Rowling said this. Grindewald echoed the sentiment of For the Greater Good, which Dumbledore at first was on board with. Until he saw how awful this would be under his friend's hand. He didn't wish to step in, as Grindewald was dear to him. But Dumbledore did his duty and helped put Grindewald in a wizarding prison in the far north. Grindewald was killed by Voldemort who was, at that time, looking for the Elder Wand.