Maybe I'm not understanding the story right, but it seems that the Shirtless Guys with Guns were just being a-holes from the start. I don't get a sense of the time period over which it happened, but the story seems to be Big Guy put mattress in dumpster, one or both of the Shirtless Guys pull it out and throw it in his yard. Big Guy later puts mattress in the dumpster again and Big Guy or his wife tells one or both of the Shirtless Guys the trash won't get picked up if it's not in the dumpster. Shirtless guys again take it out and throw it in Big Guy's yard. Big guy must have put it in the dumpster again, because it's in the dumpster in the video. Now Shirtless Fat Man with a handgun stuffed in his shorts and Bozo Son with a shotgun come out, apparently so they can pull it out of the trash again and throw it in his yard. Big Guy turns into Big Angry Guy when Shirtless Fat Man pulls gun from his waistband and starts yelling at him. Shirtless Fat Man passively-aggressively taunts Big Angry Guy, who gets more and more worked up. Then for reasons not shown on, Shirtless Fat Man shoots Big Angry Guy twice, and for no apparent reason other than the shooting's started, Bozo Son chimes in with a shotgun blast.
That being somewhat the case, I think Shirtless Guys were doing enough provoking that if they feared Big Guy would use force against them, they had a duty to retreat, even with Texas's Stand Your Ground Law, and their failure to do so means their resort to deadly force was legally unjustified.