Was his brother just about to say Paddock was an arms dealer?
It's all very sketchy. If he truly almost said "army of one," which would have fit with his previous sentences, then it sounds like he was aware of the plan before it went into action.
He was speaking about how his brother was self-reliant and did not need anyone. Thus, logic would dictate that he was about to say that his brother was “an Army of One.” Of course, saying that after he had just hosed down a bunch of people with what was effectively a machine gun, made that a very unwise choice of words. So he stopped in mid-sentence. I don’t think that he was going to say arms dealer, because that would just not fit into the train of thought that he had started earlier in that clip.
Of course, I would have found it very convenient if he had actually said arms dealer, or even just arms, because that would explain not just where he got a good part of his wealth, but also what might have happened. One theory going around was that he was involved in a deal that went sour, was killed, and that whoever killed him decided to hose down the crowd outside as a diversion to get away. Another theory is that he was acting for the FBI as an arms dealer to entrap some Isis operators, and the deal went south, etc. Note that I don’t necessarily believe those theories, but I don’t dismiss them out of hand, either. There just is not enough information publicly available right now to come to any firm conclusion.
My take on that is ‘army of one’. Then he realized he should not use that colloquial term cause Steve just killed 57 people.