I don't know if it's enough to kill the fish but 2 capfuls sounds like an awful lot for 33gal. Yes, Prime does smell of sulfur. But again, I don't know if that's enough Prime to cause the deaths.
I also did not test for kh. I guess it was the drop in ph but still can't understand why.
As for the Prime I don't think it was overdosed. I generally use 1/2 cap in 5 gallons when doing a water change so I dosed for 20 gallons only.
There have been no fish since yesterday afternoon and I kept a sponge filter and a sponge over the intake, so are those 2 sponges viable for bacteria still?
I agree the Prime probably didn't kill them. But just thought that I'd point out the Prime I have says to use 1 capful for 50 gals. Maybe yours has a different size cap or a different concentration.