Brick was only in 3 episodes towards the middle of the season last year and it was clear early on that Brother Blood was working for someone.
In season 1 you knew it was the Dark Archer, but didn't know it was Tommy's dad. In Flash you know it's Zoom, but not who's under the mask. There's no reason to make it be a twist when there's been no attempt at hiding it so far. The show's not above bad writing, but if you're going that route you're going to tease it to build up interest.
Though I'm still with you that Merlyn is a very strong candidate for who they're talking about killing, especially after last episode. But in that case it would be something personal, like the death of William, and not what the main arc of the season was.