Now, this may just be me reading it into the text, because all I have to go on for the later volumes of Yami no Matsuei is some sketchy fan translations, but I'm pretty sure there are some implications that we're supposed to read the whole set up as 'not a good thing' - something we don't get in love mode and other yaoi manga! There's implications that Enma is deliberatly keeping Tsuzuki around because of his power - refusing to let him 'escape.' Tsuzuki's the extreme case, obviously, but the same is true to a lesser degree of all the other shinigami.

As well, I think there's some buddhist influence in the set up; the shinigami guys can't be the only ones to have sufferred during their life times - in fact we are shown again and again thoughout the manga that they quite patently are not. So I read them as the ones who suffered but who were unable to let go of that suffering, and the attachment to the world that implies, after their deaths. In which case a lot of what Tsuzuki's doing during the cases can be seen as beating the system: ensuring that those who die have no regrets, and thus no reason to become shinigami. And thus being able to let go off their suffering would equal being able to let go of being shinigami - and so if Enma wants to keep them, of course the whole system is working against them ever being able to do that.

*Wry* It's been ages since I read the manga so I've no idea how much of this is textually based and how much of it my mind's added on as time's gone by to make it more platable.