A dissatisfying or incomplete experience can leave the mind churning, testing out different avenues of resolution until some closure can be reached.
Apparently the vet I saw yesterday is one of 2 at that clinic, and this one has a knack for making my wife feel bad too - even when she's taking healthy cats in for regular check-ups.
I've also found out that a typical feline reaction to a scary situation is to turn on the charm, putting forth extra effort in order to mask their unease (perceived as weakness). I didn't know this, since our other cats do it differently: one of them acts scared and skittish, crying for attention and comfort, and the other one gets aggressive, dominating and taking charge. Apparently these cats are a little weird, and the other one behaved in the more traditional fashion. This led to a sudden surge in apparent vitality, which may have raised questions in the vet (who really should have known this). It also means that her increased affection was likely for show, and she was really quite afraid - and I missed an opportunity to comfort her more specifically (I was doing much anyway, but was perhaps misreading her response).
These things make me feel a little better about the awkwardness of the exchange.
Her passing has still been unsettling, though. In order to help soothe that, and remind myself about the appropriateness of the choice, I remember her mouth. She suffered from a persistent infection there as a element of her condition. This made eating and even drinking difficult, rendering one of her primary interfaces to the world an exercise in constant pain.
She would never let me check it while she was alive. It was just too tender. The previous vet (the nice one, who managed to show compassion to both people and animals) had let us know about the seriousness of the infection, and we took his word for it and helped her out as much as possible. After she passed though, I used the opportunity to do a brief post-mortem exam; not out of fascination or morbid curiosity, more to acquaint myself with the reality of her departure and dissociation of attachment to her remains (since anything meaningful to me had moved on).
Her mouth was one massive visceral abscess. I won't get more graphic than that - it was bad. This reminds me of the degree of suffering she was in despite her best facade, and secures my sense of justification in the course of action.
It was the right thing, even if it wasn't easy.