[Jan-Poo]

what do you mean? The red marks on Natsuhi's door?

One of the general linchpins for "Shannon is Beatrice" and "Jessica is Beatrice" theories is the concept that Battler makes a sort of a foolish promise to either one of them and fails to fulfill it due to his quarrel with Rudolph

Here comes the question: "Battler made a promise to Beatrice" or "Battler made a promise to someone other than Beatrice". If Jessica is Beatrice then she might do all of this out of friendship. There's another interpretation. Shannon is doing all the killings and Jessica is protecting her by placing the blame on Beatrice. "Everything was done by the witch there is no need to blame one of us". Beatrice in ep4 did say that the promise wasn't with her.

The only problem with that is in ep5

Spoiler for episode5:

It really seems that Beatrice is the one who is in love with Battler and the one whom the promise was directed to.
But in ep4 at a certain point we see two Beatrice and one of them said to the other that she would take her place from there on. And there's also Bern in the secret tea party of episode1 saying that Beatrice is not a single person.