an itchy trigger finger
1. An inclination or readiness to shoot a gun (especially at someone) without needing much or any provocation or justification. I wouldn't stay too long around here—there are a lot of people with itchy trigger fingers in this part of town.
2. By extension, an inclination, readiness, or tendency to act or react carelessly, rashly, or without due consideration. John's itchy trigger finger is going to get him into trouble with his boss someday. When you're working in a restaurant, it's important not to have an itchy trigger finger when customers come to you with complaints.