I think the investigator was raped(?) by the criminal? It was very vague in the beginning. When the criminal was apprehended he said that he would only talk to this guy. I remember that he was kept in an isolated house and there were only three of them there, the two MCs and one guard. Everything becomes all psychological with both of them trying to keep the uppeyr hand but the investigator is more easily rattled than the criminal.

The guard is killed at the end of the 1st or 2nd volume.

The art is gorgeous

Ok, so I bought this manga online last year but I can't remember where I saved it so I'm looking for the email they sent me.

The cover of the first volume is the investigator holding a skull close to his face as if he's going to kiss it.

It's definitely In These Words.

Bless you. That's it!!!