And I agree with your argument, at least to the extent that there is always malice aforethought- an intent to kill- when the muzzle of a gun is pointed at a living target. Of course he intended to kill them. I would hope so, as that is the threat in a leveled firearm, not the firearm itself :D.
The warning WAS given, and if, as it says above thread, they were shot from a distance of 15ft, then that warning would have to be considered explicit. There is no time for parlay. Anything that is going to go down is gonna happen in a split second. ANY movement at that distance is justifiable as a threat, and at that distance, I am surprised he got off a warning at all. Your argument regarding "enough time for the warning to take effect" is incorrect for that reason IMHO.
Bear in mind that at 15 ft a gun and a knife are about an even match if the knife holder knows what he is doing- One good throw or leap, and that knife can reach out and touch you.
As for any neighbors who claim otherwise, that the guys charged him, I say that the audible evidence contradicts what they say.
Your contention aside, I would be willing to submit that the witness of folks on the ground is of more worth than anything else regarding what happened in that split second. I consider it the height of folly to gainsay what they have committed to (regardless of any motive on their part, btw).
Without casting aspersions on Mr. Horn or his neighbors, I would expect the neighbors to be inclined to provide a narrative friendly to Mr. Horn regardless of what actually transpired. That is not to say I doubt Mr. Horn was, in fact, justified, but merely that I would not regard the neighbors' testimony as having overwhelming probative value. They're probably telling the truth, but it's not hard to imagine that they might possibly be shading things a little.