Why would Casca not be a good choice as your mentor?

I assume you want more of an answer than just that he is a murderer. So, comparing him to the other conspirators, Casca would not be a strong mentor because he is a bit of a follower himself. Where others in the conspiracy - Brutus Cassius chief amongst them - show strength of their principles (Brutus's being obviously more noble than Cassius's), Casca enters the conspiracy less forcefully, letting Cassius manipulate him through the threat of the latter's suicide.