Largely agree with what has already been posted.
I'll add a few cents to the thread.
A firearms insutructor should have education, experience and training on the subject matter he is teaching. Meaning, a war hardened vet has not really gained any experience at carrying concealled. Nor has he had to operate as an individual (most of the time). Likewise, someone who has not seen combat should not be teaching assualt tactics.
Next, the instructor should be a teacher. A good teacher evaluates the student to determine their method of learning. Identifies fears, hesitation and lack of attention that might be a liability. A good teacher understands the human/firearm interaction and has the tools to bring about the proper techniques. Some of the best shooters can not teach.
The instructor must have a passion for the subject and a deep care for the student. The VAST majority have a deep care for the $. Coupled here is that the instructor is a lifelong student. Learning from others, changes in teachnology and techniques.