This should be a slam dunk for the prosecution, the only reason he took it to a grand jury is to he could wash his hands of it when they didnt indict him. You'll know this is true when he doesn't bring the charges before another grand jury and wash their hands of the whole mess.
There is no colorable argument for self defense because the men were shot in the back. Unless they have eyes in the backs of their heads and were about use deadly force against him or another person, there is simply no argument for the use of the shotgun. If you want to argue that, then fine, but it is a loser every single time on any bar exam you may happen to take.
Here's the law on the Multistate Bar Exam - The use of deadly force is NEVER allowed to protect property and may only be used if to PREVENT the commission of a VIOLENT FELONY, such as burglary.
Under the MBE, this is a classic case of murder reduced to manslaughter by reason of exciting event with insufficient time to cool off. Horn used deadly force AFTER the crime had been committed so the prevention element is missing. Also, it wasn't a violent felony because burglary is the unlawful entry of a dwelling with the intent to commit a felony AT NIGHT. This happened in broad daylight.
http://www.telegraph.co.uk/news/worldnews/1571085/Texan-%27hero%27-shoots-and-kills-burglars.html ("Mr Horn saw Miguel Antontio DeJesus and Diego Ortiz getting into a neighbour’s house at around 2pm on November 14."). There is no question that he would be guilty of manslaughter only whether he could be convicted of murder as well.
Under Texas law, one may use deadly force "when and to the degree he reasonably believes the
deadly force is immediately necessary" to prevent the commission of an act of criminal mischief (including burglary) during the nighttime AND he reasonably believes either the property cannot be protected or recovered by any other means OR using anything less than deadly force would expose him to the use of substantial bodily injury. Texas Penal Code Ann. § 9.42 (2007).
Horn is wrong on all elements. Here, the degree of force is unreasonable because Horn could have given a warning shot, or shot the men in the legs to prevent their escape. Perhaps fatal to Horn's argument is that this happened in broad daylight. Horn would not even be entitled to get a defense of property jury instruction. Grey v. State, 2004 Tex. App. LEXIS 6678 (unpublished opinion)(defense of property specifically requires the "theft" to be at night, thus no error in refusing to instruct the jury on the use of deadly force to protect property). Note that even if the burglary doesn't have to be at night, which I think the statute suggests, there were other means of recovering the property or preventing escape such as shooting them in the legs or at least give a warning shot.
What this amounts to is a political decision to protect a popular old man from going to prison. I am willing to bet they don't try another grand jury even though they have more than probable cause to arrest Horn right now.
As food for thought, compare this case with Tennessee v. Garner, 471 U.S. 1 (1985). In Garner, an unarmed fleeing burglar did not halt at the command of an officer and with his back toward the officer, began to climb a fence. Id. at 3-4. The officer, believing the burglar will elude capture, kills him with one shot to the back of the head. Id. at 4. Eventhough a Tennessee statute authorized "all the necessary means to effect the arrest," of a fleeing felon, the Supreme Court in a 6-3 decision found this to be an unreasonable seizure and the statute to be unconstitutional as applied. Id. at 11. "The use of deadly force is a self-defeating way of apprehending a suspect and so setting the criminal justice mechanism in motion. If successful, it guarantees that that mechanism will not be set in motion....A police officer may not seize an unarmed, nondangerous suspect by shooting him dead." Id. at 10-11.
This isn't the end of Horn's tribulations. He will be sued for a bunch of torts and should lose on summary judgment because, through Horn's own admissions, there is no issue of fact for the jury.