To answer your question in the scenario you present, the citizen has no legal obligation to intervene. The LEOs have not asked him to assist, and even if they did, it's doubtful he could be legally compelled to act with a suspect running around shooting at him.
"No amount of indoctrination or textbook learning will in themselves develop more than efficient mediocrity. Operations should be handled with a combination of force, subtlety, shrewdness, guile, and knowledge born of actual experience." - DF
Last edited by MeefZah; 02-07-2012 at 13:40..