He hasn't though. He has not received any knowledge or illegal activities and it's not even his department that investigates those things. The rules for fraternizing with criminals are different depending on where you live. Now if he walked in on his lover dealing drugs or killing someone then he would definitely have to report it. They don't discuss the details of either of their jobs with each other that is made clear several times.
That's the thing. He isn't bothered about what his partner could be doing. The fact that he was illegally detained to a point where he almost lost his life speak about the shady activities that his partner could be involved with. From the start of the manga, it appeared he was the typical man of principle; someone who upholds his image. But as the manga goes on, he turned 180 degrees and turned a blind eye to what could be happening.
That's why people say love is blinding
That attack was personal.. it was not because he is a detective...plus although he was told to stay away from itsuki, it was not on a professional level as well (mandated by the office). It was just done by his superior on his own... i think when it comes to principles, he didn't betray his profession..so don't be bothered ^^
Am I the only one who's bothered by the fact that the uke deviated from his principle as a detective?