I say he did the right thing. If he knew the house was locked and where the GF was, obviously that person entered illegally and did not belong there. The fact that it was an ex-boyfriend definitely raises the level of concern. God knows what he was there to do.
Based on the news story (admittedly a weak source), the shooter did not know that the person he had shot was the former boyfriend until after he had shot him.
As they say sometimes on "Best Defense," let's see how this scenario might have played out differently:
Residence has a good alarm system.
There is a power outage.
Central station calls the subscriber, but the number they have is a cell phone, and the battery in that phone has died, so Central Station notifies the PTN/KH, who is male resident's best friend from high school.
PTN figures (appropriately enough) that, since he got call from CS, his friends aren't home and there is a problem in their house, so he gets his key and goes over.
He doesn't bother the knock, as (a) the house is dark (car is in the garage) and (b) if they were home, CS wouldn't have called him.
He tries to turn on the light by the door, but, since the power is out, that doesn't do any good.
He considers going back for his flashlight, but he's been in the house many times and there is enough light coming in the windows for him to get to the breaker panel.
Which is is halfway to when his best friend from high school shoots him.
I tell my people that one of the best things one can do for home defense is wire enough lights that they can be remotely turned on from the bedroom as to light up most of the downstairs (or kitchen, dining, etc areas in a one-story).
Generally speaking, one wants to get through this life shooting as few people as one has to shoot.