Then, you should have a look at what Texas Penal Code says. The hostilities started long before the homeowner got the gun, and certainly were in full bloom after the homeowner told the trespasser to leave. God bless Texas.
Might be interesting to see if it applies. Like I said my layman's reasoning is that the property angle does not of just what you supplied. But I am not any kind of expert...just following my own reading of it. I was not trying to make moral judgements in my analysis, was just trying to zero in on if the law technically applied.
Alas, I need to start concentrating on my day job now which is all about software and not about law.
Correction...just realized you were not the one who quoted the property part of the law I was trying to analyze.
The details get a little tricky, but the best case for the defendant seems to be under Title 2, Chapter 9, Subchapter C, Section 9.31. Specificaly whether (1)(A) applies and the decedent:
unlawfully and with force entered, or was attempting to enter unlawfully and with force, the actor's occupied habitation, vehicle, or place of business or employment;
Looking up "habitation" it seems to apply to the porch in that it structurally part of the house per Title 7 Chapter 30 Sec 30.01 (1) (B).
As far as the decedent entering the porch unlawfully, I think that is obvious as he was told to leave by the land owner.
Which leaves one crucial question, was the entry "with force". Now I have not yet looked this up do to needing to end my lunch break, but it does not seem in the ordinary layman sense of the word to me he used force to enter the porch.
So I am still not entirely sure the self defense works.