Sorry if this question seems a bit morbid but i'm trying to work out what caused it.
Ignore me if i'm talking out of place.
Did it die soon after, or during the salt bath?
Do you know your tanks PH?
Really sorry to ask those questions but i think it may help me help other Betta lovers when/if they encounter problems.
I think the original cause, looking at the signs, was probably an environmental cause but (i think) the next likeliest culprit would be a bacterial infection.
I would, if you are considering a new Betta, rehouse the snails and shrimp and put the tank through a course of anti-bacterial treatment.