“Do you usually kill people you disagree with?”
Why do they do it, the villains? Why do they turn on their own people?
Dracula pushes Renfield down the stairs, Gutman’s gunsel takes the fall, Blofeld shoots Mr. Osato, Tobias Vaughn decides the Cybermen have betrayed him, Barnabas declares that Willie Loomis must die…
I mean, I get it about the blundering. You work hard to come up with an evil plan, and then all the blundering happens. It’s discouraging. But killing your employees after they’ve made a mistake means that’s one less person standing between you and the hero. Yes, you’ve removed someone that you can’t trust from your social circle, but that doesn’t make the rest of them any more trustworthy. Everybody else just starts updating their LinkedIn profiles.