One of Many Interesting Political Philosophy Questions Raised by Tom Cruise

After watching the new movie about the plot to kill Hitler, Valkyrie, Bryan Caplan asks: why are people so squeamish about tyrannicide?

I'm amazed that people who don't think twice about killing conscripts (or even civilians) are so reluctant to justify violence against serial killer statesmen. What could be less objectionable than trying to stop mass murder by killing the specific individuals most responsible for it?

If the philosophical case for tyrannicide is so strong, why do so many people - including the members of the 1944 plot against Hitler - have such strong moral qualms against it? My best guess is that (a) there is a high correlation between moral virtue and obedience to authority, and (b) political leaders are very reluctant to support tyrannicide because they're worried about retaliation and/or setting a precedent. But I wonder if that's a little too conspiratorial.