Jews an Samaritans were mortal enemies. To Jews, Samaritans were filthy untouchables - traitors and interlopers. "Good" and "Samaritan" would have been considered mutually exclusive among Jews at the time of Christ. One of the things that Christ was trying to say is that what we do matters so much more than the tribe to which we belong.
I always find it funny when people say "Good Samaritan," because it's like saying "Good vile monster."