Although ninja come to mind within popular nomenclature as the traditional Japanese agent of sabotage and assassination in the shadows, hitokiri were more prominent during more peaceful times such as the Edo Period and were more explicitly sought for the need to execute others with a greater degree of social flexibility and camouflage, as akin to hitmen; while samurai were allowed to strike down those who offended them, social standing was of utmost importance to those of higher class, and cutting down offenders demanded evidence and lawful reason with legitimate weight of action lest they risk dishonor and be forced to commit seppuku, or be punished by public execution, especially if targets in mind held major social influence and were of nobility.
Essentially acting by proxy, hitokiri in turn were not bound to these strict codes, and were of or could masquerade as ronin and lower ranking samurai, acting as the swords of their superiors and clientele. With their guise, hitokiri were able to strike within the bustle of city crowds as well as the darkness of night and the obscurity of privacy, could also join in the chaos of battle to seek out priority targets while their allies provided distraction, and upon finishing their job, could immediately fade out of public awareness without drawing attention to their superiors. Due to their tactics, their blindsiding and obfuscated nature had their acts appear as if they were the enactment of divine judgement by calamity of the everyday world, and popularly in turn, it was considered custom to leave behind notes reading tenchu, or "heavenly punishment" upon a successful hit to also demoralize and weaken the spirit of their victim's associates, and especially if another series of attacks was needed in times of political unrest or quiet yet seething civil war.
While existing throughout Japanese history, hitokiri were particularly active during the turbulent years of the Bakumatsu, when both the Ishin Shishi and Bakufu required swordsmen who moved in the shadows, silencing their enemies swiftly and disappearing without a trace. These hitokiri were frequently unknown except in certain circles, where they would often take "professional" names like "Hitokiri Battōsai" and "Hitokiri Kurogasa". Frequently, hitokiri who fail their missions and are captured for interrogation commit seppuku in order to keep their secret orders from being known and sometimes, for the purpose of keeping certain assassinations secret, the hitokiri himself would be murdered or "erased" by his so-called comrades, thereby eliminating the full knowledge of his actions from the world.
Though some consider hitokiri merely murderers, those within the profession have said of that the difference between murderers and hitokiri lies in the fact that, while a hitokiri chooses to become a killer of his or her own free will, the targets to be slain are always chosen by someone else. Any hitokiri who breaks this rule to resolve a purely personal grudge opens himself to discovery, threatening himself, his benefactor as well as his missions and will likely meet a shameful end. Many, whether outright murderers or shameless braggarts, have claimed to be hitokiri in order to make others fear them, but it is said that being a hitokiri is not about killing - Not only does when one turn down the path of a hired blade to kill for a purpose and ambition, but a true hitokiri carries the weight of the lives he's taken upon his back as they drag him down into a hell of his own making all throughout his life for that very motivation. Often, one carries the weight of his assassinations for the rest of his days and those in the know are able to tell a hitokiri by his very presence or the depth of his stare.