Matthias; Six of Crows- Chapter 20 (Leigh Bardugo)
There's nothing to gain by sending a Grisha-only unit to burn down a random village or two. Fjerdans already hate and fear Grisha, and this would only cement such belief, which is the exact opposite of the Darkling's long-term goal. Now, there are several options to explain the discrepancy:
a.) Brum lies. We don't know how much did Matthias see, so the lie can be anything from Grisha presence in general, through the level of their involvement to harmlessness of said village. When you have a chance to gain a loyal fanatic, you won't explain you made a target of their home in the first place.
b.) Not every Grisha is Second Army. Sure, Matthias calls them "Inferni soldiers", but (See: a.))... Even though we know very little about Second Army organization, Inferni work in pairs, and having a whole unit consisting of a single specialization- especially if they're supposed to be infiltrating enemy territory- makes little sense. A group of rogues would have less options to pick from if their talents don't differ.
c.) The Crown's order couldn't be ignored. If for some reason the Darkling's superiors decided Grisha are to do their dirty work, they couldn't refuse no matter how pointless the order.










