The Yehudi HaKadosh MePishischa was a Chosid of the Chozeh of Lublin, until one day one of the elder statesmen in the Chozeh's court, whom the Chozeh held to be a big tzaddik, slandered the Yehudi HaKadosh to the Chozeh. The Chozeh relied on this saintly Chosid, and that was the end of the relationship between the Yehudi HaKadosh and the Chozeh. No explaining would rectify the matter, the Chozeh would not hear of it.
Commenting on this episode the Kotzker Rebbe marveled at the power of the Yetzer Hara. For sixty years the Yetzer Hara helped this Rasha do only mitzvos and maasim tovim and helped him steer clear of any aveira. Why? Because he was investing in him for the future. He knew that one day he will use this person as a vehicle with which to cause machlokes and therefore made sure to keep his reputation pristine in order that the Chozeh will rely on him and his "tzidkus".
Sounds like a crazy conspiracy theory but these sentiments are echoed by Rav Chatzkel Levenstein. Rav Chatzkel says that if someone displays a bad middah even at an advanced age, after never succumbing to this middah in the past, surely it was in him all along. It just never had any occasion to go on display until now.
A person is born with bad middos, says Rav Chatzkel. Either you fight them and rid yourself of them, or they will lfester quietly until the right moment presents itself. That moment is not a deviation of your otherwise excellent middos. Rather it is an indication of the monster within you, that was never vanquished.