Judge, Jury, and Executioner: The job of the Fifth Directorate, where existential threats and other excessionary events are concerned. Rather strictly forbidden for everyone else (including the Shadow Fleet, Imperial State Security, etc. – although their legal procedure can, and often does, vary to fit the necessities of the cases they handle). Even the Imperial Hands are obliged to strictly follow the law and respect the rights of sophonts and citizens, even if they are generally also empowered to carry their own miniature court around with them wherever they go.
(Of course, under most circumstances, for a non-governance chap, executing someone in the course of committing their special crime is just fine, but that’s not a law-enforcement function. That’s the inherent right of all sophonts to defend their lives, liberties, and properties – and those of people nearby – against rights-violatin’ Defaulter scum, belike.
The Watch Constabulary and above-mentioned bodies, however, are supposed to try to take ’em alive if doing so is possible without inviting worse consequences. But then, they’re also trained to; it is assumed that amateurs shouldn’t be required to try and do the fancy stuff, although they can if they like.)