I don't know where to begin. You walk into a store. Steal a snake. THEN report them to the police and such??
If the animals were in such poor conditions, why not report them BEFORE??
If the other snakes were in a reptile rescue, why didn't you give the snake you stole to the rescue as well, or at least ask if you could adopt it out of the rescue?
Did you try to talk to the shopkeeper about the conditions the animals were living in and pointed out the mites to him? (Playing devil's advocate here) There is a chance this guy might not be aware of the mites, or know enough about reptiles to provide the correct care. (Again, I realize we're talking about a pet store here, so was just playing devil's advocate.... please don't hate me too much!

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Lastly, WHY, after your comments about not wanting to pay for it because it was infested and not doing well, why would you then go back and pay money for it??
While I think it's great that you were concerned about the welfare of these animals, I do NOT agree with you openly and willfully stealing an animal. You had other options available, yet you made a concious decision to commit a theft.
Next time, call a reptile rescue and let THEM handle the situation.
Take care
Annie