The grammar doesn't give us the answer. A native speaker simply understands the meaning. Part of it comes from the words and grammar, part from his understanding of his own culture. Change some things and cut it down: "A doctor who was shot at by a patient ... stopped him-1 by shooting back with his-2 own weapon, injuring him-3." The language doesn't tell us who him-2 is. (If a grammarian told me there were a rule, I wouldn't believe it!). Him-1 is the patient, We know that just from the language, in two different ways. a) "Patient" is the last person we hear about before him-1. b) The basic structure is "doctor verbed him-1" and if the him-1 were the doctor, it would be "doctor verbed HIMSELF." From the grammar alone, the most natural guess would be that all three pronouns refer to the patient. We make the unconscious guess that his-2 is the doctor. This is based on two pieces of general knowledge. First, in the United States it is not terribly unusual for people to own personal guns. Second, if his-2 were the patient, that would mean that the doctor had taken the gun away from the patient. That would be a difficult thing to do, _and the reporter certainly would have mentioned it._ It is _more likely_ that the doctor owned a gun than that he took the patient's gun away. We don't think about this. We don't work through the sentence like working a crossword puzzle. Finally, him-3. "The doctor stopped him-1, injuring him-3." Him-3 can't be the doctor, or the sentence would be "The doctor stopped him-1, injuring HIMSELF." Also, when the doctor shot back at the patient it is more likely that the doctor injured the patient than that the doctor injured himself. By the way, I don't want you to think everyone in the U.S. owns a gun. I hope none of MY doctors have guns in their office and I don't think they do. But the idea that "a doctor MIGHT keep a gun in his office" is U.S cultural background

It can be confusing to someone who is a beginner in learning English. It is definitely ambiguous and this sort of ambiguity has a technical name, "referential ambiguity". In brief, referential ambiguity arises when it is unclear who or what a word such as "he/him/his" refers to. http://wikieducator.org/images/c/cd/LESSON_FIFTEEN.pdf However, a bit of common sense can help you resolve the referential ambiguities in the sentence about the doctor and the patient. "A doctor who was grazed by gunfire from a patient in his [the doctor's] office at a suburban hospital on Thursday helped stop him [the patient] by apparently returning fire with his own [the doctor's] weapon and severely injuring him [the patient with the gun]" What are the reasons for resolving the ambiguities as I proposed above? 1) The patient is most likely in the doctor's office rather than the doctor being in the patient's office. 2) It should be obvious that the doctor stopped the patient. If the doctor stopped himself, the sentence would have had to use the word "himself" instead of "him". 3) the phrase "his own weapon" indicates that the doctor had a weapon of his own as well. If the doctor had had stopped the patient with the patient's gun, the sentence would have said "stopped the patient with the patient's own gun". 4) "severely injuring him" - the doctor is the subject of the verb "stopped" and since "severely injuring him" describes "stopped", the doctor is the subject of the verb "injuring" and the patient is the object of the verb and so "him" means "the patient".

I call it confusing, Lee. It is easier to understand if the writer used their first names. So we have to learn who 'him' is through the context.