I don't think they're evil. I think that they don't believe in Yevon, and therefore they kidnap summoners in an attempt to save their lives. Like maybe they believe that they shouldn't be sacrificed with their religion. The way they show their anti-Yevonism is a bit odd, though. Cid is just a little bit crazy and likes to blow everything up, which is not the way to go. But as you see, Rin is a very successful Al-Bhed businessman. So not all Al-Bhed are evil. ...I guess.