The Silber–George Debate
I missed the debate between Kenneth Silber and Robert A. George on whether superheroes should have to register with the government (covered here and here), but there there are potential situations when we must be careful about superheroes and supervillains, even within a libertarian frame. For example, what if a supervillain is able to control minds? In that case, a pre-emptive strike might be called for.
The stories of the German–American SF writer James H. Schmitz, who left Germany in the late 1930s, frequently featured villains with mind-control powers. I suspect he regarded it as a plausible explanation for the hold that Hitler had over Germany. Moyuscane the Illusionist in “The Illusionists” (a Mad Artist who hypnotized an entire planet) was particularly Hitlerian. A relevant quote:
"Siva Psychosis, the gentle voice resumed obligingly. "Symptom of the intermediate to concluding stages of the Autocrat Circuit in human-type mentalities—Refer to `Multiple Murder: Causes'—"On the other hand, somebody like that is likely to control the government before the government can control him.