I generally thought the same as you - it just didn't work out. Totally cool, thought it might work given the other guy being kinky and such, but it just wasn't the other guy's thing (which is cool - the only thing not cool being how he handled it, even though I also put out there that if it wasn't okay, I could be put of dynamic and such, and after his freeze-up I did apologize).

The potential other guy I know, was friendly with, but not super close to- which I considered ideal in not being a stranger but enough distance (and now with this, no problem as I can easily not run into him).

I am thinking the guilt comes from him feeling that he came up with the idea and that it fell apart after having exposed myself, so that a) he feels there is real life fallout for me (also none for him), b) his comment to me a couple of days later about the safeword and a comment earlier in the day when negotiating the rules, after discussing and agreeing, he said out of role that he'd take care of me, and c)his feeling that in this situation he failed to take care of me.

I did tell him to please not feel guilty, we were both hot and bothered with the fantasy aspect, but it had been a calculated risk and one I was okay with specifically because this guy didn't mean anything to me and that it wasn't going to affect me, and if anything it worked out for the better if this other guy was just gonna ghost and be immature rather than be direct, so it's better I didn't sleep with him (and hypocritical given other guy's own personal life). But I think that guilt is still there and he feels he lost that control of himself and didn't think clearly, so that and still feeling responsible.

I am more to happy to propose eliminating any such exposure things entirely given this experience, if only to avoid bad feelings and look after him (as I think Ds and ss both look after each other, in their own ways) and prevent that regret from popping up again. Though I could always, in discussion, float that idea of finding someone else that would be a better fit as you say by putting the dynamic front and center from beginning if he still wants to experience this.