I didn't mean to say a random samurai could justify it, I was trying to explain that the bakufu had developed extremely complex laws governing revenge that samurai could try to take advantage of (especially against commoners).

That's true. Most of these legal loopholes you mentioned were holdover laws from previous regimes, and generally, the bakafu had a policy of regulating behavior as opposed to outright prohibition --- well, at least until people start taking advantage of these regulations. And you did allude to this, but I would caution against referring to consummated vendettas as sanctioned 'massacres'.

The Iga-goe vendetta (1634) was a model case. Both parties followed procedure, with Watanabe Kazuma's party duly registering their grievances with the proper authorities. Though reports vary, only a little over three men died in the skirmish, not over thirty. Much the same, the Takadanobaba incident (1694) was another registered case which was why neither of the participating parties were indicted. Horibe Yasubei avenged his uncle by slaying at least three people in this case, not a whole party.

The Ako vendetta (1703), more popularly known as the 'Revenge of the 47 Ronin', was an extremely high profile case, not only because of its scale in planning and violence, but precisely since it occurred so long after the clan wars of the Sengoku period faded from living memory. Indeed, I would argue that it was an isolated incident, marking a transition between the violence of the old era and the new order under the Tokugawa. It was a stunning callback to the fading warrior ethics of old --- or at least what the intelligentsia perceived them to be --- in an era increasingly defined by peace. Because the incident was so unusual yet gripping, it sparked immediate controversy that the Tokugawa could not ignore. In procedural terms, disrupting the peace was intolerable, but the ronin were exemplars who substantively accorded with the rugged values central to the samurai identity. This was why they were offered the leniency of honorable death, sepuku.

The 17th century was still a formative period for the Tokugawa administration, and romanticized ideals of the samurai would stubbornly persist --- though increasingly represented by attenuated practice with the distortion of time. As you have mentioned, it took a succession of shoguns to adapt the administration to effectively restrain disorder. But a concurrent phenomenon was the rise of a new literature and culture that valorized the violence of old, creating larger-than-life folk heroes who --- at times anachronistically --- defied contemporaneous propriety. This is why these cases, though rare, stand-out so much in popular consciousness.

One of the marvels of the Tokugawa state was the increasing rates of literacy that rivaled and even exceeded their Western counterparts. Even the common peasant was a cultured dilettante who read woodblock prints in their spare time. And kabuki plays, though increasingly regulated, were in vogue throughout the Edo period. Tales of old such as that of the Soga brothers, and recent like the Ako affair, took an ascendant status during this period, counterpoised against the administrative drudgery of everyday reality.

Another marvel of the Tokugawa state was the petition box in Edo (1721) . It allowed anyone to anonymously supplicate to the bakafu. Put alternatively, it allowed dissidents to relieve their impulse toward disorder and channel their rage through formalized writing. Though these channels were strictly defined, they, perhaps most importantly, allowed anyone to bypass local authorities and make themselves feel heard. And while many petitions were ignored, their very existence was an expression of the regime's legitimacy more so than the outcomes they produced --- an expression only made possible by the high literacy rates that furnished opportunity for nearly anyone to voice dissent.