To go bad, means to have a
bad trip.
A person having this experience is in extreme discomfort, often being sure of dying within the next moments.
This phenomenon can be caused by, but is not limited to: taking too many drugs, combining drugs,
depressive thoughts.
Bad trips can be prevented: never use new drugs by yourself and moderate your drinking when you smoke weed.
If you see someone who is
going bad, try to comfort them by distracting them. More often than not, this occurance is harmless and merely a thought, that the person needs to let go off.