I really liked a lot of things about Rogue One, but in an otherwise quite grim film (especially by Star Wars standards) K-2SO was exactly the right sort of darkly comic relief. Also, I immediately pegged him as Marvin, but with the depression replaced by sociopathy.

My head-canon for this is that somehow the Sirius Cybernetics Corporation exists in the Star Wars universe, and very, very briefly had a contract for droids, because K-2SO certainly has a Genuine People Personality™.