The real issue is that Texas law, unlike just about every other state, explicitly allows you to use deadly force if:
So in Texas you can use deadly force to prevent a person from escaping after committing aggravated robbery during the nighttime. Note that if this had happened during the daytime, he probably would have been charged with at least criminally-negligent homicide (which is one of the charges he was up on). But since it was nighttime it came under Texas law PC §9.42 (Use of Deadly Force to Protect Your Property) and the grand jury declined to indict. The local DA said he (Earls) had three possible defenses: Mistake of fact, defense of third person, and protection of property at night. The DA also said that the robber will be charged with felony murder if caught.
Most other states don't allow the use of deadly force to stop a fleeing felon (unless you're LEO).