Moroni at the start of this chapter is angry with Ammoron because he (Moroni) knows that Ammoron knows that he is not justified in his actions of war against the Nephites. This knowledge that Moroni now has, this assurance of Ammoron’s fraud, (which he already had, but now having it expressly presented to him) changes the course and approach of Moroni in how he hopes to liberate his captive people. Instead of hoping for some peaceful compromise, he instead uses strategy to secure their liberty.
The interesting thing about Moroni’s anger, which is clearly a motivating force throughout this account (see Alma 51:14, 54:13, 55:1), is that it never causes him to lose control.
Take home: if you know the difference between right and wrong, and wrong is being presented as right or acceptable, then there is need to take action to correct that which is wrong, not in uncontrolled confusion, but in intentional clarity.