Bill is a strong guy so he could hold his own if he got into a punchup fueled by the knowledge he has been tricked. I guess a 1,000 angry tigers would not be able to stop his puches if he sought revenge.
E9 did say he was struggling emotionally with what he had done to Bill. Might this be the nicer side of E9 having a guilt trip, and wanting things to be special for Bill as well as for Allie?
I think E9 is just evaluating the whole situation, because we don't really know if Bill has a breaking point as his marriage continues with Allie. I think Allie truly does love Bill, because she makes a hug effort to comfort him everytime E9 has fucked her at least twice while Bill is watching.
If Allie was a heartless bitch, she wouldn't give Bill that level of comfort, and it could be, that as she holds him tenderly with a mothering instinct as well as a feeling that she is boss, she is purring inside with the contentment that she has both men needing her body and her love. That must be great for her self-esteem, particularly as her mother has been shutting her out as she grew up.
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What I would like E9 to write about in future chapters, is his progress with Allie's mum - because that is where we find out who is Boss of that relationship. Might he make things happen so we can all write him a thankyou note if he did?