I agree. Seems the first rule of self defense is to not needlessly place yourself in danger in the first place. I think he overstepped his bounds and responsibilities. He knew who the man was, he was perfectly capable of identifying him, and he knew where he lived. Once he knew all those things he should have simply made himself available to testify once the man had been apprehended by the police. I think he should be charged with manslaughter.
The Supreme Court has disagreed with your presumptions and your conclusions. There are a number of self-defense cases dating from the 1890s. One case, Alberty v. United States, 162 U.S. 499, had some similarities to this case. A man witnessed another man climbing into the window where his wife was staying. There are others: Wallace v. United States, 162 US 466; Allison v. United States 160 US 203; Beard v. United States 158 US 550; among others.
We can't say that the house was where the suspect resided--he was leaving the house--but assuming he had just raped someone, would it be beyond him to trespass on another's property? Assuming he had raped another, would you or any other reasonable person believe him capable of other misdeeds? The Supreme Court cases cited justify his actions.
That's not what the story said. It says that he observed the man, who fit the description of the rapist/woman beater (long haired red headed men being a not all that common, except in certain circles) running into a nearby house, probably around the corner by the address and location given. He didn't know if the man lived there or not and he didn't know who the red headed stranger was. He then observed the man exiting the house and moving away. He asked him to stop and wait for the police. He did not pull his weapon until the man began fumbling in his pockets for something, which indeed turned out to be a pistol.
So the only thing that might constitute "overstepping his bounds" was trying not let the thug who beats up on women get away, by observing him, and asking him to wait for the police to arrive. The thug is the one who went for his gun first, although he botched it pretty badly, and won't be beating up any more women. His girlfriend *probably* will be beaten again, as most women who pick one violent loser will likely pick another one. But lets pray she doesn't.