Yes, this debate is going around in circles. To answer your other questions, it is precisely because the shooters couldn't have - and didn't - know what the intentions of the would-be thief were, or whether or not their lives were in danger that they (the shooters) do not have a valid defense of self defense. Look at the statute. It requires that someone be under the reasonable belief of impending death or great bodily harm. This requires a showing of more than just "they might have wanted to attack, maybe." It requires a showing of an actual physical assault likely to lead to death or great bodily harm. No one has said that such a thing happened. If you don't believe me that this is the standard, look up the case law yourself. You'll find that I'm right.
Moreover, in the eyes of the law it is irrelevant that the murder victim was armed. What matters is a) the shooters did not and could not have known that at the time, and b) even if they had known, the guy wasn't actually attacking them with the weapons at the time he was shot. If shooting someone who happens to be armed (even though they're not attacking you) was valid in the eyes of the law, I imagine quite a lot of you would re-think your position on carrying concealed weapons.
You may wish the law operated differently. I'm sure many people would agree with you (although not me). Nevertheless, under the law as it stands, and based on the information we actually have, these guys committed murder.