Gone South

I hear this far more often to mean “gone bad” as in spoiling food or as in negotiations or relationships that started promising and later deteriorated.

to lose value or quality

When oil prices went south, it caused problems right across the economy.

She played well in the tennis championships, except her serve seemed to have gone south.

Originally it was a phrase applying to the life of a slave.
Say there was a house slave in a state like Missouri, and he was not performing or behaving as was expected – - he would be sold to a slave trader who would take him to the deep south to be sold to a plantation owner. Chances are that the slave's life would be much harder and probably shorter in the south. When the slave's friends would ask his family about him, they would reply that he had “gone south”.
Gone South is not so much a euphemism as a metaphor.

This is an incorrect definition. “Gone south,” or “go south” is a euphemism for a deal going bad, be it an illicit deal like drugs or stolen property, or a bad monetary investment. A deal “going south” can mean that one is caught by the authorities, or the buyer of seller, whichever party is not you deciding to rob or double cross you.

Alternately, someone “going south” indicates their leaving you when in need. For example, if two friends were in a physical confrontation with others and one ran off leaving you to fend for yourself, it would be said the fleeing party “went south.” This can also apply to any situation where assistance was expected but the other party left, or failed to help.

I have heard these usages countless times in film and in conversation. I have never heard this used to mean “died.”

To Go South, was to be Sold South. Slaves that couldn’t be controled were sent to Richmond Va., sold and boated down the James River to be sent to the deep South.