This statement is true in real life as well as in the movies. Here is a list Reddit has chosen of heroic people that lived long enough to become the villain.
Regrettable_tattoos posted, “Prolific sex criminal. He put a public face of goodwill and charity. However, anyone who came forward was ridiculed. Those who knew (was an open secret in the industry) buried any mention of it.”
Jyunga said, “Kevin Spacey would have gone down as an acting legend.” Another user replied, “I hate it because he was genuinely one of my favorite actors after The Usual Suspects, Se7en, and American Beauty."
Baloo88 posted, “He was possibly the Continental Army's best battlefield general. He led the Americans to victory at the pivotal Battle of Saratoga, where he was wounded. Had he succumbed to his wounds, the Northeast US likely would be covered with statues of the man.”
Evilkumquat posted, “Stephen Collins from 7th Heaven was accused of being a serial child molester. He was never arrested, just investigated, but he admitted in an interview to the acts.”
Savdqst posted, “Bill Cosby? He was always pretty bad, but people considered him a hero.” MasterCotCot replied, “He was nicknamed America's dad for a while. Insane!”
One Reddit user said, “He went from global inspiration after winning multiple gold metals running as a double amputee to convicted wife murderer.” ChefAtRandom replied, “Fun fact, he only started winning after getting his prosthetics adjusted to be longer.”