This is a generalization to show that many people participated in the event. Alternate translation: "A very large number of the people of the land" (See: [[rc://en/ta/man/translate/figs-hyperbole]])
Treason is acting to overthrow the government. Athaliah had killed the legitimate heirs of the king and made herself ruler in [2 Chronicles 22:10-11](../22/10.md). She was angry that Joash became king in her place. The exclamation is repeated for emphasis. Alternate translation: "You are committing treason!"