Been a lot of talk on this, all I can say is that a Grand Jury decided to indict him (issue a "true bill") after reviewing all the evidence. While I understand his reaction, his use of lethal force was not justifiable under Texas Penal Code Section 46. Murder in the absence of self-defense is never allowed, either morally or legally.
Remember that all the Grand Jury does is determine if probable cause exists to try someone for an offense. It is not an insinuation of guilt.
If he has a good defense lawyer, he will claim extreme emotional distress and temporary mental incapacity. I think that will be sufficient in getting an acquittal.
I have deep sympathy for the father as I would probably have the same reaction given the circumstances. I hope he walks; while I don't agree with his actions, I most certainly understand them.
As for the drunk driver, he got what he deserved, no matter how it was administered...