Moroni had recently fought a battle against apostate Nephites, a battle that had cost many lives. The threat from Amalickiah is yet another form of apostasy, but this one is an internal threat rather than an external one. It is unsurprising that Moroni would be angry with someone who was attempting to do internally (to destroy the foundation of liberty) what Moroni had just done externally (in defending liberty).
The banner that Moroni made was an impromptu creation. It was intended to be seen, and easily raised above the heads of the people so that they could see it even when they were in a group. The fact that he wrote on it requires that we think of what it might have meant to have writing on the banner. First, it would be expected that someone could read it, else there was no reason to write on it in the first place. Nevertheless, the writing itself would not be seen by all. If one saw it from a distance, or the banner were fluttering in the wind, the words would be difficult to read. On top of those issues is the probability that literacy was limited.
Therefore, the writing became part of the symbol. It would be read to others, and the meaning of the words passed around, but the function of the writing would be that it be seen, and not necessarily be read.