#1: The Manhunters

How deadly are they? They destroyed all life within Space Sector 666 because of a "programming glitch." How big is a Space Sector? Unknown, although the entire universe is split into at least 3601 of them, and Final Crisis claims that there are "thousands of worlds" within Earth's sector. So let's just say that's a…

#2: Unicron

How deadly is he? He eats planets for breakfast. And lunch. And dinner. And if they're populated, all the better; it adds texture, apparently.
Who's responsible? It very much depends who you ask; Unicron has been given multiple histories throughout the years, including being a fallen god who somehow became a robot, a…

#3: The Cylons

How deadly are they? They destroyed the twelve colonies of humanity, committing genocide on a scale unimaginable to everyone except Glen A. Larson and Ron Moore.
Who's responsible? It depends on which version you're asking about. The original series had the robots built by a dying alien race also called cylons,…