TexasCHLforum.com

I believe that RoyGBiv has the key point correctly. Animal are considered property under Texas law as far as I know. Therefore, look at Sections 9.42, 9.42, and 9.43 for the legal justifications for using force. Animal cruelty is not one of the crimes listed, so the best you might be able to get to is that it would be considered criminal mischief, so if it occurs at night then you might be legally covered.

And the other half of this is that all property in Texas is owned by somebody. All animals are owned, either by some individual or by the state. If the person is killing his own dog/animal, it would not even be considered criminal mischief. And I sincerely doubt you could prove that the third person who owned the property asked you to protect it (by shooting him).

All in all, I think you might be able to win in a jury trial if you used some force, but not deadly force. And I am not sure I could recommend banking on a sympathetic jury even for that.