I'm not an expert by any means, but if what you're saying is true, then it sounds like you did everything right and the snake may have already been sick when you brought it home. Its hard to say for sure without an autopsy though. I would buy from a reputable breeder if next time if you didn't do so this time. I have had luck at expos and most of the breeders there stand by their animals because that's their livelihood.
Anyway, I hope this helped.