Well there is no set evil. As Gaius and Kilgharrah have said before, magic is how you choose to use it and does not determine if one is evil. Morgana believes she is fighting for justice and that Camelot is the evil (she has only killed people from Camelot before, notice she doesn't hurt anyone out of it). We haven't even really seen her good side since Series 2 as she's always involved in plotting against Camelot. As for Mordred, he is sooo ambiguous. I think the events in The Disir will determine how evil he is. Perhaps while he was in a coma he saw a vision of Arthur condemning him to die? Anyway, there is definitely more to Mordred, as predicted and the producers said he has a role to play at the end. He wore the Saxons logo in the trailer. Considering dark music tends to play when these characters are on-screen, though, from a general perspective I guess he can be called evil, as classification. (He is listed under Series 5 enemies on the BBC website) It just depends on how you look at it!