I think DOC likes butchering pretty much, there was a crazy nurse in Acting Lessons and try to kill everyone in the game.Right now, I consider that something that's supposed to scare us off or it might be another red herring, it's mentioned just once and there's no mention of it at all after that (possible implication that MC does not survive the ending of Being a DIK?)
But let's delve into the different versions of what it might be about:
Bella is my favorite LI in this game and I would absolutely hate it if we don't get a happy ending with her (not necessarily Bella and MC riding off to the sunset together - MC helps her move on and years later they encounter each other by chance and MC sees that she finally did move on and have a normal life, married and created a family of her own.)
- Bella suffered a miscarriage or had a stillborn at one point and her husband left due to the stress, grief and strain that such a traumatic experience causes. This explains why Bella is so cold and isolated towards most people. Also, it's a logical guess of what's behind the locked door - a nursery and after James left Bella chose to lock the room to isolate the memories (but I can't give an explanation to the boots seen beneath the door). Also, it might explain why she thinks that she "forgave him years ago" while in the pool with the MC. I believe this version the most.
- James is/was a soldier and he went away on a mission and is gone for a few years, with absolutely no news about his status so he might be MIA. This is believable and an implication might be made from the mannequin the MC runs in to while drunk. The locked door was James' home office and Bella locked it to isolate the memories. This version however, offers no explanation of what Bella forgave James for.
- James cheated on Bella with someone and she caught him in the act and kicked him out. Very weak possibility, I don't think she'd still be wearing the ring if she caught him cheating.
- Bella cheated on James with Jill (as it was implied in that scene in chapter 5) or someone else and was caught by him and he left in anger and didn't come back. Possible, but difficult to explain - it leave too many plotholes. I think Bella and Jill did become intimate with each other at one point after Jill was comforting her one night and Bella swore it wouldn't happen ever again.
It's not wired that Bella might be a killer in the end. the boots seen beneath the door might be the body of her husband(haha).