go bad - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Spoil, decay; also, turn to crime. For example, You can tell from the smell that this milk has gone bad, or If he keeps running around with that street gang, he's sure to go bad. [Late 1800s]