Implausibility handled poorly comes across as stupid. Handled well, it becaomes a source of drama and a potential plot generating device. The demon may be in the details of the way she plays the character (no pun intended), but I like a lot of others here see nothing particularly impossible about the concept. It's actually rather intrigueing.
I once played a Drider Paladin myself. Had a lot of fun with him, figured he had a crisis of faith when he was transformed, and opted to try a completely different path in life. Think, born again, in more ways than one. So, for me the fun was playing a character that was totally devoted to a noble cause, but who had never really known good in his early life. He held fast to certain basic principles, but he had to learn the details over the course of the campaign. The tension between a completely evil background and a conviction to do good gave the character some depth. Once he figured out what was good, he was absolutely fanatical about doing it, but there were often moments of hesitation while he tried to figure out how to go about it. All the better considering that he still had a poisonous bite ...a real source of temptation when things got tight.
Whether I succeeded or not, you'd have to ask the other players, but the idea was intriguing enough, I still think it makes for a great starting point.