----- SPOILERS AHEAD ------
Alright theory time for chapter 1
So for the motive, thanks to my little grey cell I came up with the conclusion the abbots are one way or another helping demons appear to not lose ground in the world and make money, frog guy let us know financially not much is going their way yet the boat stop before reaching the church island let us know demons are appearing too close to the church and they make big money of it, the documents may prove this activity and clearly would be important enough for the abbots to risk their lives over it.
Now for the murders I'm leaning toward Remigio since he's the one person the guards don't have any issues with seeing out at night so with him being the murderer both ben and santiago testimonies for night 1 hold since the murder happens past midnight aloys testimony still hold and only give him an alibi up to midnight.
Second murder he may have stolen Santiago's master key and tipped the victim about the safe location at the same time he gave them the master key, obviously I'm also having to say he stole the 2 church keys on top, one for himself to enter from the church main entrance and to swap with the master key post redrum and the other to make it look like no master keys were used ( my assumption would be he might know of it since he's been at the abbey for so long, and as he claims, everyone's gramps so one of the confidant may have told him someday )
Murder 3 I think his way to get into the abbots room is by baiting with books !I think that because him and aloys train at the library, he'd likely know about the fact that oscar gets books delivered to him, he may even be the one that prepared the package and intentionally didn't provide all the books on the list to have a way in Oscar's room. The fact that the missing book hasn't been found yet lead me to believe he is still in possession of the files and may be trying to keep the remainings abbots at the abbey longer.
And now for my last bullet, Octavio Spiner