I also know past and present law enforcement people, some have had to shoot people. (note they volunteered the info, I did not ask) Even those with training shake due to the adrenaline rush when drawing down on someone. I expect I would be even worse then them, making it hard to stay on target, to say nothing of the guilt I would feel for taking someones life.....justified or not.
One guy I know went into great detail about what it was like, the smell of the spilled blood and all. Not something I would look forward too and would do what I could to avoid it....including looking like and being called a coward. But, that does not mean I would not fire if circumstances were forced upon me.