I’d say that lethal force is rational when used as proportional self-defence. Or, in the words of Malcolm Reynolds, “if someone ever tries to kill you, you try to kill 'em right back.”
I think the Gandalf quote, however, is specifically referring to death as a punishment for “just” being a thoroughly contemptible, evil and heinous person.
I would still classify what you’re describing as proportional self defence, since punching a fascist is always self defence haha 😄