How many underlings did Vader kill?
I can only think of that underling that Vader force choked, whom, if I recall correctly, questioned/mocked the Force. (So kind of justified.)
Other villains killing underlings tend to be examples of an otherwise smart villain having temper control problems, so the first time we see it, it's probably an exception, but yeah, if it continues through-out the story we should see the minions abandoning ship.
When the modern take on the Joker started killing minions (The Joker's Five-Way Revenge) it was novel, but later writers kept doing it so it's reached the point where one wonders, "Why does anyone willingly work for him?"
Phil Foglio's Girl Genius has the novel idea that minions get caught up in the master's enthusiasm, but whoa to those who disappoint the minions.