#6: Sentinels (The Matrix)S

How deadly are they? They took over the Earth and keep humans around only as batteries. Which, you know, is potentially an environmentally solution to the whole need for energy (Although they did start out with solar power.
Who's responsible? Humanity en masse. The robots that ended up taking over the world started out as domestic help and cheap labor to handle the jobs that we didn't; it was only after it was decided that robots had no legal rights that they decided to get nasty.
Last seen... coming to peaceful terms with what was left of humanity on a pretty-much destroyed Earth in The Matrix Revolutions.

Next: Time-Traveling Murder Machines!