1) You need to be aware of the situation around you. What we don't know is how the guys that were following your friend were acting prior to the attack. In my opinion you can usually pick up on someone acting in a way they are a potential threat.
2) Shooting someone for just taking property is a real judgment call. If I understand the Texas law correctly that is one of the things you can draw your gun for. However, shooting someone as they flee seems like it can have legal ramifications.
3) I bet the odds are against you for drawing on someone who already had the "drop on you".
In my opinion, if you didn't pick up on my first point, using your weapon in this situation is kinda remote. That said if the BG with the gun came after you shooting and missed...............then it becomes a different situation completly.
Just my opinion.
