Mafia 93. In the Name of the Moon! ☾

I didn't quite understand the part about Ment's murder. Was he killed by the Mafia? The pen and the dog movie – those hardly seem like two clues, so it seems like one of them is a hint about the killer's role?

The pen is part of the role description. I think it means that Makoto-Ment has already been initiated as a warrior, and in the event of Usagi's ultimate ability, he can be resurrected. So, the clue is the dog movie.