An apostate is someone who has deserted his cause.
The word apostate originally comes from a Greek word that meant “runaway slave.” Now, apostate has a religious or political tone to it, so someone might call you “a political apostate” if you ran for office as a Republican during one election and then ran as a Democrat in the following election.
not faithful to religion or party or cause
not true to duty or obligation or promises
a disloyal person who betrays or deserts his cause or religion or political party or friend etc.