If you are in fear of death or serious bodily injury you can. Add to that threat, the grabbing/attempted grabbing of Kyle's gun by both aggressors that were killed, no doubt self defense should be allowed. It would be reasonable to think that both aggressors could have taken his gun and used it on him.
It's little details like this that show prosecutor misconduct. They are literally trying to convict an innocent young man. Nobody in their right mind, who saw Kyle getting whacked, believes their sob story.
Depends on circumstances. Is the one being chased in fear of death or serious bodily harm? Does the state law allow for self-defense in that situation? That's why there's a trial. The prosecutor knows this, but he's obviously driven to ignore the facts and video, running this circus trial for some agenda.