I think a reasonable person, with no intent on harming you, would either communicate their reason for approaching or walk away when confronted verbally (e.g. "STOP! Don't come any closer") as many have suggested. In both of these stories, the "bad guys" did not do so. They did not act like most people would which gives the "good guy," and any reasonable person, "reasonable ground to apprehend a design on the part of the person slain to commit a felony," and therefore justifies his or her use of force (in Washington).
The second story is less clear and I think the good guy had better options, such as driving away, but I think he would have been able to use that defense (whether successfully or not depends largely on the prosecutor, arresting officers, jury, etc.). I think it is wrong to suggest that anytime you are the first to use force you are not acting in self defense. The law does not require you to be injured before you act. It only requires, as quoted above, that there is "reasonable ground to apprehend a design on the part of the person slain to commit a felony..."
