While violent punishment such as drawing and quartering, being flayed alive, braided on the wheel, and the brazing bull may sound like justification for such evil acts, a quick and simple bullet to the head to dispatch the vermin without another thought would be my choice. Then deposit the remains in an unmarked grave. If public and violent execution was actually a deterrent there would only have been one hanging, or one head on a pike. The evil need to be quickly killed without ceremony so their true punishment may begin in earnest.
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