#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 murderous robot exiled from his home planet or the much more common "his origins are lost to the mists of time." Given that all of the Transformers' origins are a little nebulous (Someone had to have built the first Transformer, right? But who?), it's possibly best not to dwell on this point for too long.
Last seen... Being destroyed by Galvatron (of all robots) and his body folding itself into a black hole called The Unicron Singularity in Transformers: Cybertron.

Next: Science Fiction's Most Killhappy Robots!