So does that mean that he faces the possibility of being charged under the old law (that he would have applied to everyone else if he had had his way), rather than gaining the benefit of the new law?
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus
It would, but he didn't violate the old either. Man was on his property, very likely trying to steal something. That's "theft during the nighttime", which is one of the justifications for use of deadly force under Texas law, old or new.
The relevant section of (the old) Texas law is:
§ 9.42. DEADLY FORCE TO PROTECT PROPERTY. A person is justified in using deadly force against another to protect land or tangible, movable property:
(1) if he would be justified in using force against the other under Section 9.41; and
(2) when and to the degree he reasonably believes the deadly force is immediately necessary:
(A) to prevent the other's imminent commission of arson, burglary, robbery, aggravated robbery, theft during the nighttime, or criminal mischief during the nighttime; or
(B) to prevent the other who is fleeing immediately after committing burglary, robbery, aggravated robbery, or theft during the nighttime from escaping with the property; and
(3) he reasonably believes that: (A) the land or property cannot be protected or recovered by any other means; or
(B) the use of force other than deadly force to protect or recover the land or property would expose the actor or another to a substantial risk of death or serious bodily injury.
Of course you can also use deadly force to protect yourself or another from death or injury, of course with a "reasonable man" standard as to the perception of the potential for death or great bodily harm.