This might sound a bit scattered, but I just finished re-reading 'Death on the Nile' and the mastermind thing really stuck with me this time. It's Simon Doyle, the husband who gets shot in the leg. He and Jacqueline de Bellefort planned the whole thing together from the start to murder Linnet and get her money. The brilliance of it is how Agatha Christie uses him as the seemingly obvious, wronged party—the guy who got shot trying to protect his wife. That performance, the timing of the 'attack,' it's all a perfect smoke screen.
What makes Simon so effective as the villain, though, isn't just the twist. It's how his apparent simplicity, that almost puppy-dog charm, hides the cold calculation. He plays the victim so well even the other passengers buy it. You almost feel sorry for him until Poirot pulls the threads apart. The plan hinged on everyone seeing Jacqueline as the unstable, jealous ex and Simon as her target, not her partner. It's chilling when you realize the 'love triangle' was a facade for a much uglier financial plot.