There's a good answer to that question here:
http://www.law-forums.org/what-s-the-trespassing-law-in-texas-t9798.html
The links cited there state that
force is allowed to protect property, but
deadly force is not. I am guessing that your attorney would advise killing the trespasser with your bare hands, rather than a gun, so that you could argue that you didn't realize you were using deadly force. But either way, you are still subject to Federal laws like civil rights, which are independent of a state's decision to not bring charges.
There's a thousand years of legal history regarding self-defense law, going back to the English common law that the US adopted, and all of it says that force is not acceptable to protect property. But the new laws that we are seeing in states like Texas and Florida are starting to lean in the opposite direction. They're trying to codify the prosecutor's discretion that has always existed.
In this case if the guy dies, I would expect his relatives to sue the homeowner's insurance company for about a million bucks. They'll offer $100k immediately to settle, but would probably pay a quarter mil to not have to litigate. If you didn't have insurance, they would just take whatever assets you have, possibly the house itself if there is equity.
Killing people tends to be a bad idea in a lot of ways. And thinking that you have the right to do so is a very dangerous attitude if you make the wrong decision.