I need some advice on an unfortunate accident.

I had a cat that was an outdoor only cat. I wish I could've brought
him inside but I had no control in that matter as my parents wouldn't
allow it. Recently, the cat was attacked by a dog and as soon as I
heard the commotion I ran outside to get the cat. He was alive laying
on his back with his paws in the air. I immediately rushed him to the
emergency vet hospital. They said there were no puncture wounds but
his back legs and tail weren't moving and that he was paralyzed. They
took several xrays and determined there was no spinal damage, no
internal bleeding that they could see, and everything else looked ok.
His bladder was empty and he did have stool in his colon. They said it
was possible that damage could've been done to the bladder but weren't
sure. They had no idea what was causing the paralysis and it could be
shock or that damage was done to the spine. I think they were
suggesting that the dog could've picked the cat up and should him hard
enough to cause damage to the spine even though it was fine on the
xray. They also said his temperature went from 96 to 94 and they were
trying to warm him up but his temp. was dropping. It's possible he
could've already had a low temp from just being/living outside. They
gave him what looked like 0.3 to 0.6mg of Buprenorphine, 4mg of
Dexamethasone, and 200mg of Ampicillin. They wanted to keep him
overnight but one of my parents had a cow about the cost and insisted
the cat was simply in shock and would be fine in a day or two.

So I took the cat home against my better judgement. About 40min to 1hr
later I noticed him breathing strange and moving around a lot. I
decided to take him back right away. Unfortunately, he died before we
got there.

I asked the vet if there was a possibility that he would've died even
if I had left him there. They couldn't answer that but did say they
could've slowed it down. They also said that the cat had a bowel
movement shortly before it died. I think it happened before we left
the house and that was part of the reason he was moving around in his
box so much and was so distressed.

I am upset about what happened but I think it was something that was
supposed to happen, it was predestined. I won't say why but I just
have this feeling that everything was already mapped out and nothing
could've changed what happened.

One of the things bugging me the most is that I have no idea what the
medical reason was of why he died. The vet didn't seem to know either.
They had no idea what caused the paralysis, no idea why his temp. was
falling and no idea of what caused him to die. I know it's not really
important now, but I'd really like to have some idea of what happened.
It just seems odd that if the xrays were normal and there was no
internal bleeding that could've he died so soon, unless something was
seriously wrong.

Does anyone have any ideas as to what might have been wrong?

The dog that bit him, had bit a person a while back and the person
said that there was no puncture wounds (just a scrape). However, they
ended up with a gigantic bruise that hurt very badly with a sharp
stinging pain. It was at least 6 inches wide and 6 inches long and was
deep purple. A few months later there is still a dull pain and either
a light brown bruise there or some sort of permanent skin damage that
leaves a light brown color (they're not sure which). If the dog bit a
person that hard for only a few seconds and left that kind of damage,
I wonder what damage it could've done to a small 9 pound cat that
maybe wasn't visible with an xray.

So having said all that, I would like to hear opinions about this
situation. I'd like to know if anyone has ideas on why the cat died. I
realize I'll never know for sure but if something like this should
ever happen again (it better not), I'd like to be prepared.