Had a police shrink tell a class once that it is a quite common dream to have confronting a situation where deadly force is needed and you feel like the weapon has a thousand pound trigger pull of some other malfunction.
She credited this with the inner conflict most of us have to both the huge amounts of stress involved and the need to take a human life. I had these dreams a couple of times a year as a rookie until I had been involved in some major violent incidents and then they stopped. Don't let them bother you to much, and keep on training. In the end its your training mindset that will take hold.