General Depravity:
* Go to the dogs: This idiom means to decline in quality or morality.
* Go downhill: This idiom means to deteriorate or decline.
* Take a turn for the worse: This idiom describes a negative change in someone's character or situation.
* Fall from grace: This idiom suggests a loss of moral standing or reputation.
* Go astray: This idiom refers to deviating from a proper or virtuous path.
* Lose one's way: This idiom implies losing sight of one's moral compass.
More Specific to Depravity:
* Turn into a monster: This is a strong idiom that suggests a complete transformation into a cruel or evil being.
* Become corrupted: This idiom implies a gradual erosion of moral principles.
* Sell one's soul: This idiom indicates a complete surrender to evil or immoral desires.
* Be consumed by evil: This idiom emphasizes the overwhelming power of darkness over someone's character.
The most appropriate idiom will depend on the specific context and the nuance you wish to convey.