Debate #196 update: The grand jury will not press charges against the father who killed a man he found allegedly sexually abusing his daughter. Texas law provides several justifications for the use of deadly force, including sexual assault. 
I think someone just found a way to eliminate due process, and was able to act as judge, jury and executioner - in this case it was literal.
I can really only imagine a few rare cases where lethal force would be required to stop a sexual assault. The guy beat someone to death with his bare hands, clearly showing he'd lost control. After he stopped the sexual assault, he killed someone.
Since the law shows precedent, that case would be shown as an example in lots of other court cases to instruct the judge and jury how to make their verdict. How far is someone allowed to physically assault an 18-year-old boy sleeping with a 16-year-old girl? Somehow I don't think it would be justified in beating an 18 -year-old girl who slept with a 16-year-old boy. What if someone finds evidence that their child was being sexually abused, and beat a person to death despite no assault taking place at that time? What if a child was receiving emergency medical attention and the adult administering the help was beaten to death because at a glance it looked like possible sexual assault? The jury could come back and refer to the case you mentioned, and say the intent was exactly the same.
I said before in a previous answer about this case (before the grand jury made a verdict) that the law takes into account the situation to plead down sentences. My suggestion was that the guy should be charged with something like aggravated manslaughter ("aggravated" in this case meaning he was instigated into killing someone), which would probably sentence him to 3 years in jail, which would probably see him out of jail in a year after being judged by a parole board, and with the option of being given a house-arrest buy-out. You have to admit, being under house arrest for a year for killing someone is getting off really easy.