How can Oedipus consider himself responsible for evil actions, given that he was unaware of the evil nature of the actions at the time when they...

This is a very good questions and engages two important
issues in the history of philosophy and theologyu. The first is that of free will and
predestination: if the gods have determined Oedipus' fate (by the curse laid upon the
house of Atreus) how can Oedipus be considered responsible for his actions? Second, are
acts judged evil by their intentions or their
consequences?

One way to think through these questions in
terms of ancient Greek religion is to consider the importance of "miasma" or ritual
pollution. A person who offended the gods by committing acts which caused him to become
unclean or ritually polluted brought the anger of the gods down on the entire city.
Oedipus' killing his own father and marrying his mother both make him ritually unclean;
thus the gods send a plague on the entire city until this pollution is cleansed.
Oedipus' self mutilation and resignation of kingship purify the city (see Oedipus at
Colonus for the final resolution of the Oedipus story). The issue isn't so much evil in
the modern sense, but pollution and its effects on the city.