1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): go bad
to become rotten, undesirable, evil, etc. • I’m afraid that this milk has gone bad. • Life used to be wonderful. Now it has gone bad.
McGrawhill's American Idioms And Phrasal Verbs