M was dying, and not a field operative. Kincade was an old man who was not prepared for his life to be threatened by anything other than old age. Considering that they were scared, inexperienced, and (in M's case) mortally wounded, I don't have a problem with them behaving erratically.
Hell, trained soldiers have been none to freeze up or behave erratically when they come face to face with an actual firefight.
The movie wasn't without problems, but this wasn't one of them.