I could try this -.-
Spoiler:
This theory is probably wrong because I'm not really in the mood to thoroughly check the red statements....
Kanon is the culprit and the murderer. Jessica is the accomplice. Jessica locked the door with battler and kanon inside. Now, either Kanon told Jessica to kill Battler (let's just suppose he's sleeping), or Jessica found out that the one they were going to murder was Battler. (and I'm also supposing she has a good bond with battler).
Upon discovering this, she did not like the idea and was trying to get out of the situation. Kanon did not agree, they start discussing, and in the end, Kanon ends Jessica. Simple as that. --'
OR, and this one is quite nice in my opinion... xD It's a love triangle. Jessica locked the door of the room were Battler and Kanon were lovey-dovey, and assaulted Kanon trying to get hold of his cleaver :P Kanon doesn't hesitate, and in a flush of anger kills Jessica.
by the way, this sentence:
The one assaulted Jessica was neither Kanon Nor Battler
If it's not wordplay, then it still makes sense because Kanon was the one who was assaulted.
If that is not right,I'd ask to recite the following:
- All characters are awake and physicaly healthy during this whole thing.
- Jessica shares a tight bond with Battler.
- Jessica is friends with Battler.
- Jessica loves Battler.
- Kanon loves Battler.
- Jessica and Battler are lovers.
- Jessica and Kanon are lovers.
- Kanon and Battler are lovers.
- All characters know each other; they are not unknown to each other.
- Jessica is not the accomplice of any form.
Just wanna make sure that it's not a love thingy. :Either way, I think the only thing left to figure out is the motive. We pretty much know who the culprit is by now..