User blog comment:HandsomeBanana/Brandish magic theory/@comment-177.107.9.64-20150709191748/@comment-24367018-20150709192730

It's not possible to say this after only two chapters of her being in the series, as well as this, how do you explain the blood splatter when Marin went pop? Furthermore, who said anything about criminality, the Albareth Empire is stated to be a militaristic nation, pragmatism is always abundant within the military.

So from there it is possible to see that Brandy took the pragmatic approach to avoiding a fight with Natsu and co. by getting rid of Marin which seemed to be the simplest option. It also allows for Mashima to get out of dodge by finding an easy way to beat Marin after giving him overpowered magic.