Sarcastic to a fault is not right
You do something thats good to a fault...like he is honest to a fault . It basically means that you are nothing but honest. But you wouldn't say someone is sarcastic to a fault.
I'm confused as to what "to a fault" means. If I say, "He's sarcastic to a fault," does that mean he's a little too good at being sarcastic so that it turns into a bad thing?
Nothing wrong with the 2 answers given so far, but you might get a better feel for this idiom here:
To a fault references the highest level .. Like for example: He is honest, 'to a fault' .. Means he is very honest, without a doubt.