There's lots of things that can cause a cars battery to die but a common cause of the battery dying especially if the battery goes dead overnight when your vehicle is parked is a bad alternator diode.
A vehicles alternator could test out fine charging but a bad diode in the battery can cause the circuit to continue charging even when the engine is shut off and the alternator is not charging.
The diode circuit being closed and charging can drain a cars battery overnight.
Other things that can cause a battery to die overnight or when the engine is off is something left plugged into the 12 volt system such as cell phones, or a short in the wiring harness.
If none of the above is the cause of the battery dying then your battery could have a bad cell in the battery which could be causing the cars battery to lose it's charge and die.
I would take the car to Autozone or another auto shop and get the battery load tested and the alternator tested as well.
A regular auto shop can have more testing equipment and can give you a better diagnoses on why your battery is dying and could tell if you have a short in the wiring harness as well.