IT gives you unease feeling because once again is trickle truths.
The fact he panics about that chat means he knows full well something significant was there, enough to delete it and fear you may ever come to see it.
And I imagine he does not want to tell what was about.
From the sound of it I could speculate is about "his deep love for the AP, how he was doing everything in his power to leave you for the better woman (the AP) and live happily ever after, how they are soulmates and unique and how they plan life with children and all kind of limerent crap".
nd he could be either bullshit or limerence when he was into it. That's what could be so terrifying into you finding out.
Now it is understandable, if he is truly remorseful he does not want to lose more respect into your eyes by showing you the naked truth of how he conducted the affair. He might be afraid that will break the back of the camel and you will finally leave him.
That's why he deleted immediately.
So it is understandable. In part.
on the other hand he is again choosing to defend the Affair and the Other Woman rather than defending you.
Retraumatization? Nah, not that. You are too fresh into it, you are STILL traumatized, it will take years of hard work for you to truly begin healing that wound. Actually he still keeping secrets and hiding the sordid truth from you is a blockage to you healing from betrayal trauma.
Because as you feel the sting now, you will always remember about this final lie that he refused to confess to you.
In short, once again, you are not chosen, like during the affair. This time again he is choosing himself and the OW.
Now the question is: is he doing this maliciously?
Maybe not.
If as you say he is truly committed to R and guilty and remorseful, in his flawed character he might truly believe he is "protecting you" (because that chat was real shitty and he remembers it very well), but in reality he is protecting himself, his ego and the affair partner. At the cost of your peace.
WS need to make the work to understand why the BS needs to hear the truth, ALL truth, even the disgusting most revolting parts of the affair.
Because hearing that is painful and crushing for the BS, and will pour salt into their wounds. But, and is the least understood but, the truth is the critical ingredient to rebuild the trust that was broken by the betrayal.
You will suffer when knowing the content of that chat, be sure, because he is guarding it so much it will be hurting you terribly.
But the mere fact he will disclose his lowest detail of the already low betrayal, means he is displaying to you the most vulnerable, shameful part of himself for you to see, judge and decide what to do.
Showing you that horrible shit means he trust that no matter what you will do, you deserved to know, and he will accept the consequences.
And this will help you to rebuild trust (after pain).
Not knowing save you from the immediate pain, but your mind movies will spin it for the rest of your life, maybe making it even worse than it truly is. You will not have peace, he is making sure you can never reach it.
Lies and secrets are no good for R.
Try to explain him, hopefully he will understand.
YEs it is a matter of transparency, you are right.
Without it you are building the new foundation of R over explosive, and if you don't take it out it will blow you both up someday.