Most people would not call Oedipus evil. To be sure that he killed his father and married his mother, but he did all of these things unknowingly. More importantly, he has a heart to help his people. In this sense he is noble. Even when Thebes is suffering from a terrible plague, all he wants to do is to solve this problem, because he wants to be a good leader.
As the story unfolds, he does not know that he is actually the problem. In fact, this is the furthest thing from his mind. So, it is no surprise that he is angry at Tiresias and his words that suggest that he is the cause the problem. In Oedipus' mind, he is not the cause, but the one who want to solve the problem.
Finally, when he comes to realize who he really is, he accepts it.