Ugh, I know what you mean about how some places raise & keep their rats. It sounds to me like the 3-4 minutes required for the rats to expire was NOT long enough for
their "internal guests" to do the same, & that's how your snake got worms.Poor snake...(not to mention their other customers!)
When you say you will no longer be feeding pre-killed any longer, I HOPE you don't mean you're switching to live? I HOPE you mean you'll be using frozen-thawed???
And with f/t, no parasites can survive, so it's much safer, though your snake may balk at the difference in taste & texture...I hope he'll accept them again.
I had pet rats long before I ever got into snakes...but sadly, not every source raises rats (& other rodents) to the same standard of health & cleanliness. I know that from
dealing with pet stores in the past when I supplied them with rodents & what they told me about their previous suppliers...the quality difference can usually be seen in
the rodents too, if you know what to look for. And as far as our snakes go, it's "garbage in, garbage out"...even if you don't breed, you want healthy food for your pet.