It’s a legal shooting, but the fact the guy left the scene is suspect, and probably illegal.
Not illegal. It's not like a traffic collision, there's no legal duty to stick around after a legal defensive use of force, at least in Texas. And the fact that he left, while it might be used against him to "impeach" his testimony is the least of his problems, and might even be good tactics. Frankly I'm giving serious consideration to that very thing in my own ROE's.
The problem that the defender is going to have is justifying the "reasonableness" of his use of deadly force for the shots following, at minimum, the 4th.