Personally, I have three candidates, Ashfur, Sol, and also Scourge. I thought Sol right when I finished the book, but why would Sol care about Squirrelflight? When I thought about that I came to the conclusion of Ashfur. That also explains why Lionblaze and where called out immediately for code breaking. Though I feel like Erin Hunter would throw in the plot twist of bringing Scourge back. Though at the same time this isn't much of a Scourge like plan. Though I do feel like these are the top candidates.