In no particular order... Not sure why it matters whether or not the deceased had a gun - lots of damage to be done with fists. Was it mutual combat? Sounds like the shooter had some carry knowledge, since he waited for police to arrive. From various reports, I get the impression that the shooter knew that he might be walking into a threatening situation - prudent thing would be to leave (or not go there in the first place). What does this all add up to? IDK
Because, you are only allowed (by law) reasonable force, not deadly force, to defend yourself.
It escalates to deadly force if you are stopping a forcible felony, or preventing serious bodily injury. (Of course, also to terminate unwanted entry into or attack upon your house or occupied motor vehicle)
The problem will be explaining to a jury, or prosecutor.
If you are 95 pounds dripping wet, with multiple serious medical issues, facing an attack by 3 violent felons, all over 6 foot and 250 pounds, well your case is probably going to go good for you.
But if things are closer to even, well a jury, sitting in a heated/air conditioned room, most of whom have not made life threatening decisions, well your case might be in jeopardy.
So you're assuming because they were both teenagers, that the person being attacked should be expected to be able to respond with his fists? That's a major leap. I'm not experienced with physical combat, and I've got a few miles on me - any physical threat, weapon or not, will be treated as a threat of severe bodily harm. But as for the kid explaining it to a jury, well...