The people’s reaction is quite understandable. Their king and queen are unconscious, surrounded by equally unconscious servants. With them, also unconscious, is a Nephite—by definition an enemy. It is not surprising that the Nephite should be blamed (he was, in fact, responsible) and that the situation should be interpreted as “a great evil” (which it was not).
Culture: An unexplained detail is that the people could identify Ammon on first sight as a Nephite. The traditional assumption of a difference in skin color is almost certainly not relevant here. (See commentary accompanying 2 Nephi 5:21.) Ammon was definitely known by sight to a handful of other servants, but would the rest of Lamoni’s court have seen him before? Perhaps not, and likely the ordinary townspeople would not have seen him, since he had been either in the palace or en route to and from Sebus. Almost certainly, however, everyone knew of his impressive feats. I hypothesize that the townfolk could identify everyone else in the room by sight; the stranger, therefore, would have been Ammon the Nephite by default.