why was there even a grand jury involved?
It's standard procedure in Texas. Every homicide is reviewed by a grand jury.
This point is a prominent discussion in every CHL class I've attended: when you shoot someone in self-defense, they may die.
If they die, you'll have your actions scrutinized by a grand jury. You should plan on spending $10,000-$20,000 on a lawyer just to get through it, even if your action was completely justified.