Basically they just don't really care about anything
"John was apathetic to poor people"
"They were mad that the person they met was apathetic"
to vent to someone, about any personal issues (or just a vent itself)
"Hey, can I vent to you about something?"
"I really need to vent to someone"
For someone who can't control their anger, no long explanation.
"That guy over there has anger issues, try not to make him mad."
"I have anger issues, sorry."