On the law.
In Texas, you can use force or deadly force to defend another under the same circumstances you could use force or deadly force to defend yourself. So basically, you are talking about a reasonable belief that the other is in imminant danger of losing his life or sustaining severe bodily injury.
With a little further reading, I figured out that the DA has to prove - beyond a reasonable doubt - that your fear was NOT reasonable. And in this case, if you say you thought the guy was trying to murder the victim, the DA would have to prove - beyond a reasonable doubt - that he wasn't. If the DA couldn't do that, the jury has to find that your fear was reasonable. (That's sort of an extended Castle Doctrine that Texas has).
So imagine that you are a member of the jury. And say all the evidence is that video. I'd conclude that he probably was trying to kill the guy. And I don't see how anyone could watch that and conclude any less than he might have been trying to kill the guy. Either way, that's a win for the Defendant.
And I don't claim any special knowledge of this law. I can just read and write. So talk to your Texas lawyer who does criminal law before you decide to get aggressive.