User blog comment:B214/Chapter 501: Mari and Randi/@comment-130.105.140.154-20160912120742/@comment-24367018-20160912121350

Dimaria's just watched Brandy refuse to do anything to Alvarez' enemies, and when in a position of undoubted superiority Brandy still didn't kill Natsu or Lucy. Dimaria has likely known Brandish for a while and would have seen how ruthless she could have been in the past.

As Invel stated when the 12 were just being introduced, "to go against the word of his Majesty is to betray the very name of the 12." Zeref's orders were to wipe out Fiore, allowing Lucy and Natsu to survive goes against them. And as we've seen, refusing to follow orders within Alvarez is tantamount to treason.

Treason is almost always punishable solely by execution, therefore Dimaria's actions are in keeping with her epithet and maintaining the integrity of the 12.