How the hell can you know what the other person's intentions are? You can't ... you can only make an assumption of their intentions ... and your ability to make such a judgment is greatly affected by the circumstances including your emotional state of mind if, justifiable or not, you feel threatened, and how quickly you must do so.
To be clear: I'm not defending the shooter; rather, just making a point on your comment.
I’ve read numerous accounts from the story on FR and two things required a long jail time. He shot the kid twice including once when he fell to the ground. Another was he said, “don’t come back”.