I am so sorry to hear about the loss of your beloved cat, Frisky.
We can only speculate as to what could have happened to Frisky while he was away, and I agree with everything Opus said. It's never easy to make the decision to euthanize, even when we know it's really the best and only option that can be made. It's harder when the decision is thrust upon us as suddenly as it was thrust upon you. But, as hard as it was to hear this recommendation from the vet and as hard as it was to make the decision to allow the vet to humanely euthanize Frisky, know that you made the best decision you could under those circumstances.
My heart goes out to you as you grieve the loss of your precious and beloved cat, Frisky.
I am so sorry for the loss of your dear Frisky....we aren't Vets on this site, only pet owners, you'll have to rely on what you were told after his examination. the third eye showing is a good indicator of pain, but I can't tell you why the swelling?
to show up so quickly (within 36 hrs) seems unlikely to be a cancer, these lymphomas are usually quite slow growing.
We always feel the guilt after a decision to euthanize, but I rarely hear of a Vet who will recommend this unless they feel its warranted.
RIP dear little Frisky♥