So I'm thinking I might switch the 7th Seal quest to another more neutral quest line, and making that simply a personal side quest for Epsilon. However, with that, Epsilon will likely join another group, unless BE decides to leave or make a new character. There are enough people who have asked to join to make this a thing.
If this happens, and you do get a insidious character like Epsilon in a good/neutral party, I do expect you all to apply by the rules, even at the cost of slightly metagaming their personalities or editing the character slightly to allow coexistence (at least within the party). If a time comes when betrayal is necessary and appropriate for the narrative, either the campaign will end, the betrayer(s) will leave to join another group or become NPCs, or other alternative tbd.