Hmm, in general what Gabriel said about not judging people is right, but it doesn't apply quite so much when membership of the group is something you choose. Sure, there can still be good magi, but the fact that they willingly follow the Association's rules and philosophy does generally imply they're probably not that great a person.
Rider was wrong about Mille, of course, but the vast majority of magi are assholes, and Rider knows all-too-well how bad they can be.