He offered to take it back and give you a refund, right?
Then it's your decision to either take the refund, or keep the snake. What YOU do with the snake has nothing to do with the deal with him.
And frankly, snakes DO die. When you have a LOT of snakes, occasionally one will die. Just because he'd not weeping and wailing over it in the email doesn't mean he "has no regard for the animal's welfare".
His customer service is lacking, especially since he shipped a different animal to you than you ordered. Other than offering to take it back, what else would you expect from him as a seller? He already stated the snake he did have on hold died, and IMHO, he should have contacted you and told you that. But at this point, what other resolution would you want?