What were the writers thinking?

They are failing on all fronts... It might have been self defense, but her brother will never believe that just as he will be blind to the fact she is a traitor until it's too late. I see what they are doing; they are setting up a tension between Grace and Claire's brother in an unnecessary arc. Grace will need detective Carter for protection and just when she needs him the most he will in some kind of conveniently contrived way learn and accept the truth that Grace really did kill his sister so they want audiences to think of the possibility that he might turn on her in that defining moment. Would make for good drama if it wasn't so predictable.

However, I think this arc pales in comparison in terms of stupidity to the Liam subplot and how much of an idiot he is being. They're trying to make him into a turncoat where he regards Resist as heroes now all because they want to implement his idea for the asteroid. If they had gone with another idea over his then he would have fallen right back into the fold with Darius which only illustrates how much of a co-dependent sheep Liam is not capable of thinking for himself.