A part of me wants to say its Murder in the first degree. The act of shooting them was clearly premeditated, proved by him saying "I'm gonna shoot! I'm gonna shoot!" Self defense cannot be considered because they were shot in the back, so they weren't threatening the guys life.
Link to Texas Castle Law
Now, according Section 2.A.3, anyone can stop someone from committing robbery. Section 2.E can be interpreted to mean that if someone has a right to be where they are, they are allowed to act.
So, according to the Texas law, the man is innocent and the grand jury made the right decision in dropping the charges.
Morally, I believe the mans beliefs are justified, but the execution (no pun intended) of said beliefs are wrong.