Now that's a very interesting premise. The first undercover guy got so high on his own shit that he went on and became an Oyabun for real, and now the MPD is understandably pissed off at him and it is sending another guy to assassinate him. Not even apprehend or indirectly take him down, literally just send in a guy to do black ops shit. The question here of course, will Gorou walk the same path since it is obvious that somewhere along the way Sou aka Yoshitaka got corrupted by all the fucked up shit he himself had to do, and finally gave in to the temptation of power. Even if Gorou succeeds it is likely he will be tempted to become the next Sou as well. This reminds me that Donnie Brasco - a real life infiltrator of the Mafia - had to be taken off his assignment a bit early by the FBI, and some gangsters who associated themselves with him got hit as a reprisal for letting a fed spy in. The official story was that the FBI thought things were getting too dangerous because Donnie was asked to participate in a hit. But, and this is just a hypothetical, what if the feds did it because they knew he was starting to enjoy the job a little too much? This gray area is basically what this manga is all about.
Proper hard boiled story if you ask me, great stuff from the author.