#10: Mechagodzilla

How deadly is he? He rampages continually against Japan and fights Godzilla on a regular basis, with his laser eyes and flamethrower breath. I'm guessing there's got to be some level of collateral damage going on there. Also, he kills giant monsters, which can come in handy.
Who's responsible? Originally alien monkeys the Simians (who built him as a tool to help them achieve world domination), but humanity keeps finding itself rebuilding Mecha over and over again, apparently forgetful of the fact that he keeps on destroying parts of cities.
Last seen... disappearing to the bottom of the ocean to ensure Godzilla's death in 2003's Tokyo SOS.

Next: Robot Warriors!