Seems reasonable to me. There may be more to the story, but as of now it looks like the teen was caught in the yard stealing by the home owner, and then was shot after he threatened the man.
Read the comments for some interesting discussion. It seems that the overwhelming opinion of Texans is that getting shot to death for not respecting someone's property is perfectly acceptable.
I mention to the dissenters who suggest that the old man should have just casually summoned the police to deal with the trespassing thief, that the trespassing thief, if he was really innocent of stealing, could have just casually summoned the police to get his things back instead of heading towards the homeowner on his own property in a menacing manner. Especially in Texas.