I imagine the thing about 'if they twitch shoot them' may be more applicable than you think. If a person with a pistol had just car jacked you; and inside the car they made a move that you honestly believed was a prelude to them shooting you, we were perfectly entitled to shoot them. Is that the case in the USA?
First, you are right the differences between jobs and countries play a major part in how you act. As far as the twitch goes, it depends on whether you are law enforcement dealing with a felon using a weapon ... in some cases in that world it may apply, but for citizens to go past escaping the danger is dangerous here in the court room. If your kids were still in the car, or wife, innocent, etc. then the rules change for civilians as well if they are trying the save another from serious bodily harm or death.
Here, it so depends on if you are in a liberal area such as California or New England, or a more Constitutionally guided area such Colorado, Texas, or even Florida. The results from actions may not result in like outcomes in court ... sad but true!