Many good thoughts here in the above posts, maybe we need a "Should you carry thread?"
Proper mindset and will to use deadly force is important issues. Many find it easy to hit the target on a range and yet have trouble in on the street situations. I haven't seen too many cases of it being the other way though in my experience.
Things that can affect accuracy can be from various small things that can add up.
Some examples are:
It may be easier to shoot an animal then a person if you have strict religious beliefs and in the back of your mind you think you may be "murdering" someone.(will to use the force needed.)
Incident occurs when you least expect it (defensive mindset not working at the time). You may be thinking about grocery list etc. when walking into a convenience store not looking for possible suspects etc.
The attacker may not fit your preconceived notion of who you think is dangerous, could be a different color, sex, ethnicity of who you expected or even their age. What if the attacker looks like one of you kids or grandparents? This could delay your response or cause you to miss.
What about if you have the right mindset or will but you have not practiced with new weapon, ammo or carry position? Then you have lighting conditions, have you shot in all lighting setups and weather or clothing layers?
The suspect may not move the way you anticipated, many folks don't realize how fast a self defense situation occurs. And how few shots are fired on average.
As mentioned above you will not be sued or criminally charged in most cases of shooting an animal or target but you should be mentally prepared for legal issues if you use a weapon to defend yourself regardless. Many LEOs are trained to play a "What if" scenarios through their brains while they are on the street so they are mentally prepared to use force with confidence and accuracy if needed. Not to make them paranoid but to prevent if possible any delay in using the amount of force necessary to control a situation so they can end their shift and go home to their families. There can be a delay in your response if you don't know the legality of when you can use deadly force and when you can't. So I encourage everyone who carries to do their homework and understand their responsibilities along with their rights to carry.