It's got to be Irina. 🕵
Remember what she said to Naughn "You’re not as covert as you like to think"?
Irina knows everyone's secrets, but no one knows hers. The blessing she gave to him marrying Sydney was fake. Irina knew that she was going to have to kill Naughn sooner or later. She didn't like him as Syd's boyfriend, especially with the way that he kept bringing up that he hated her for killing Bill. It seemed like she merely tolerated him.
Irina is the criminal mastermind.