trigger-happy 
adjective
Ready to react violently, especially by shooting, on the slightest provocation: territory controlled by trigger-happy bandits...
- Regan gasped, surprised, not expecting such a fierce attack from his cousin, though she was known to be violent and trigger-happy.
- It exceeded the 4 x 6 inches allowed by the jail, so I was pleasantly surprised that it was not rejected by the trigger-happy mailroom.
- Johnson saw the risk posed by trigger-happy reporters and presidents mistaking passion for facts while dispatching others to march in enemy crosshairs.