How deadly are they? Apparently, an unstoppable killing machine that continually causes trouble for the Republic and their Jedi forces during the many years of Star Wars' Clone Wars. Despite their apparent ineffectualness, the war continued for many years, therefore the estimated high kill-rate and higher ranking on this list. Sorry, prequel haters.
Who's responsible? The dully-named Trade Federation Army and Confederacy of Independent Systems, who seceded from the Galactic Republic and then tried to convince others to do so with the help of trigger-happy idiot drones who like to kill things. But let's face it, it's all really Senator Palpatine's doing, considering he was the one pulling all the strings behind the scenes in the first place.
Last seen... standing down after Darth Vader killed the Federation's leaders in 2005's Star Wars Episode III: Revenge Of The Sith.