Against a 65 year old guy, a muscular 17 year old guy (from the picture in the article), even unarmed, is a mortal threat since he likely can beat an old guy to death. If the old guy is in bad physical condition, even an 18 year old girl can be a threat.
Once they're shot and on the ground, though, the plausible threat is greatly reduced. He should have just held them at gunpoint and called police.
He shot them both in the hip before they got down the stairs. So he didn’t know or care who they were.
The first shot in the hip might have been self-defense, but the succeeding ones were most definitely murder.