Until the modern era, courts had little power to enforce their authority, so they instead declared criminals outlaws:
When it was originally imposed in the Middle Ages, a sentence of outlawry was essentially a death sentence that the court did not believe it could immediately enforce, largely because the person was nowhere to be found. An outlaw was a person whom it was illegal to give any food or shelter, and whom it was legal to kill on sight as one might a wild animal. The pronouncement caput gerat lupinum, “Let his be a wolf’s head” (gotta be a lycanthropy storyline in there somewhere) set someone outside the bounds of civilized society. The theory was that a person who failed to show up to answer a felony charge was admitting their guilt.
Still, by the modern period, the definition of outlawry had shifted somewhat. Sir William Blackstone, perhaps the most famous English jurist in history, had already observed by the late-eighteenth century that while outlawry was still a potential sentence for criminals, it no longer permitted an outlaw to be killed at will. Rather, it permitted anyone to arrest them for prosecution and retained the penalties for aiding an outlaw.
The spread of civilization and thus of court authority reduced the need for declaring someone an outlaw, but a world full of supervillains might still see the need:
If the Joker is threatening to drop a bus full of school kids off a bridge, yeah, Batman can do whatever, because deadly force is justified in preventing the deaths of others. But if the Joker is between capers, private actors, like most superheros, can’t go after them without exposing themselves to civil and criminal liability for wrongful death, impersonating an officer, false imprisonment, excessive force, etc. But if the supervillain were declared to be an outlaw, hey, all bets are off. Go nuts.
Of course, the whole notion of outlawry is completely incompatible with modern concepts of due process:
For starters, declaring someone guilty because they failed to show up for trial violates just about every procedural standard it is possible to name, chief of which is the presumption of innocence, an important civil right enshrined in Coffin v. United States, 156 U.S. 432 (1895), which held: “The principle that there is a presumption of innocence in favor of the accused is the undoubted law, axiomatic and elementary, and its enforcement lies at the foundation of the administration of our criminal law.”
So that’s probably out. But trial in absentia is probably out too, at least without some changes being made to the law. Federal Rule of Criminal Procedure 43 requires the presence of the criminal defendant, and Crosby v. United States, 506 U.S. 255 (1993) makes it pretty clear that a criminal who is arrested but escapes or absconds before trial cannot be proceeded against until he appears in court. It may be possible that a trial might be permitted to continue if a defendant is there at the beginning and then voluntarily leaves (see United States v. Lawrence, 161 F.3d 250 (4th Cir. 1998)), but simply bringing charges against someone and going to trial without them would probably not be permitted under current law.
Still, if I were the prosecutor, I’d make the argument that a supervillain that law enforcement is unable to apprehend but who damn well knows about the prosecution is a different case than your standard, underprivileged, uneducated, minority offender that the legal system is right to try to protect from being railroaded or lynched inside the courthouse. One might even limit the definition of “supervillain” to “one who possesses powers or abilities so far in excess of ordinary human beings that forcefully apprehending them would either be impossible or almost certainly cause significant loss of innocent life.”* This might constitute a violation of equal protection, but there’s a good case to be made that such an approach would survive strict scrutiny, in that it is 1) directed at a compelling governmental interest, 2) narrowly tailored, and 3) the least restrictive means of accomplishing said interest. Given that a challenge of this sort would probably be first attempted against a notorious villain the cops are unable to apprehend, and that a sentence of outlawry would manifestly assist both superheros and traditional law enforcement in their efforts against said villain, the pressure to find some way of carving out an exception would be pretty significant. Then again, hard cases make bad law, so whether or not this is a good thing remains to be seen.
One cynical commenter notes that slapping the terrorist label of a super-villain should be close enough to declaring them outlaw.