I cannot think of any single word that means a person's life changed for the worse. While there are many phrases, i.e. fallen heroes for good guy turned bad, or take a turn for the worse, turning to the dark side,I cannot think of a single word that describes this phenomenon. An example would be in my subjective opinion, how Lindsey Lohan a child star took drugs as she grew older or Harvey becoming Two-face in Batman.
The sentence structure should be: He/She __________. And they would get the idea that their life turned bad or took a change for the worse.
Best Answer
There are many single words that mean change for the worse, such as decline, deteriorate, regress, degrade, fade, weaken etc. A good thesaurus will give you more.
But to fit precisely what you are looking for, I think the best expression is go(ne) off the rails. this may be used more in Britain than America. But if you look in Cambridge Dictionaries Online it has an entry on it.