Agree with Toad. I've taught a number of CCW classes, and in addition to what he mentioned, you legally and morally are obligated to retreat if you are able, so long as it won't jeopardize you or another innocent parties safety.
Yeag is right too, shooting somebody in the back in a non-battlefield situation is poor poor form. Also, is somebody is stupid enough to give you their back, you should IMMEDIATELY be following the Time - distance - cover rules of a gunfight and be moving to better/some cover, as well as doing a tactical or emergency reload of your weapon if required.
I hope this guy gets off, I'm no fan of B and E, but IMHO technically he is guilty of the very least of using extremely poor judgment.
The flip side of the argument I guess is that in the dark he can argue that it's very hard to tell if his opponents were armed (not sure if they were, didn't read the articles), and back shooting them when given the opportunity could have been a "oh my god I was scared they were coming back, or getting friends, or more/bigger/some guns from their car etc". He still should have moved to cover first and re evaluated his situation. Then he wouldn't be in this mess.