If Mormon was perhaps bothered that Chief Captain Moroni had used a stratagem in battle (see Alma 43:30), he was certainly appalled at Amalickiah’s duplicity. The amazing thing is that Lehonti accepted Amalickiah’s offer. Somehow, Amalickiah made himself appear to be sincere and honest; although, how one might consider a general who turns on his own army to be honest, is unknown.
Mormon includes this story both to show how Amalickiah gained the influence he ultimately did, and also to demonstrate the untrustworthy and evil nature of the man.