confusing

Disclaimer: this is a long read. Sowwy.

Prosecutor wants a cop that will do anything and break any rule to catch criminals. That cop is the mc. He hates criminals and will harm himself and cross any line to catch or kill them. He doesn't gaf.
Also, the prosecutor wants the cop to be a pawn that he can use and discard when/if needed. Again, that cop is the mc.
The prosecutor is exploiting his trauma to use him to catch criminals by any means neccessary.

The mc knows this and is "fine" with it because he doesn't value himself and is super suicidal but high functioning. That's why he said to the ml he willingly walked into hell. That hell is the dark path he is on (being a bad person and a bad cop) and his twisted arrangement with the prosecutor.
He's allowed to do anything it takes and the prosecutor will cover his ass legally and professionally.

The prosecutor is allowing the mc to murder criminals without consequence, act without warrants, probably allows him to breach a bunch of police code of conduct procedure. The prosecutor is allowing him to act outside of legal and professional bounds.

I don't know if I agree that the prosecutor is obsessed with the ml. He's just a prosecutor (with a personality disorder or something). The ml is at the top of a major criminal organisation. It's the police's job to arrest and charge him. It's the prosecutor's job to ensure the ml will be convicted on those charges if the case gets to court. The prosecutor is a twisted fucker, so he's willing to do immoral and illegal things to secure his conviction. But that has nothing to do with the ml specifically. If anything, the prosecutor is obsessed with convicting criminals, kinda like the mc is obsessed with catching/punishing criminals.

Mc pretty much says as much too. When we see the flashback of them alone in an interview room, the prosecutor shows his quirky side and mc basically says "you're fucked in the head...me too though twin".

If you meant why is the prosecutor obsessed with the mc (the cop), same answer. The mc/cop is just a means to an end - a tool to be used. If anything, the prosecutor seems to not care about other people in general. He has expressed multiple times that he doesn't understand why people behave the way they do when love and affection is involved. He isn't obsessed with anybody (that we've seen so far) . Everyone is a pawn to him, they all serve a purpose and they can all be discarded. He only cares about doing his job any means neccessary.