NOBODY, can give you the right answer to your questions as there are too many crazy people out there, even if you give them your money they will shoot you. If you run away they will shot you. If you fight back they will shoot you!
What to do then?
It depends on the situation and the guy in front of you, also the location where it happens, by that I mean, if you are in a country, where a human live is less worth as that of a cow, you should be thinking twice to fight back. You should also avoid appearing as a potential victim... don´t try to show off with chains, expensive watches or acting as a prototype of a tourist (with camera etc. or in a safari outfit in south Africa - the natives are not running around like that around too right?)
Is the guys in front of you nervous? Anxious in any way, maybe under drugs? There are many situations where people got shoot 'accidentally' because they were scared respectively surprised by the attack of their victim and shot him.
Running away from someone that has a gun? Seriously? Just give them your belongings and obey to what they tell you and don´t make any quick or suspicious moves and the most important - stay calm!! Staying calm (or at least try as hard as you can) enables you to think.
If they are after YOU personally, the chances that you have done something wrong too, are pretty high and then it´s your own fault - live with it, it is the risk if going the wrong path in live. And even then, it depends ... shooting first?
you got my point... ?