#5: Skynet/The Terminators

How deadly are they? They took over the Earth and are pretty much trying to destroy all human life, starting with a successful nuclear annihilation of three million people. You've seen The Terminator movies, right?
Who's responsible? The military. Sure, you could blame original creators Cyberdyne Systems, but I'm blaming the bulk of the problems on the US military, who took over the Skynet project when Cyberdyne was destroyed. They wanted to create the ultimate defense system, after all; why didn't they foresee that it would decide that they were the threats that it needed defending from?
Last seen... making life difficult for John Connor and friends in Terminator: Salvation.

Next: Ineffectual-But-Deadly Droids!